Merge tag 'dm-3.16-fixes-3' of git://git.kernel.org/pub/scm/linux/kernel/git/device...

[karo-tx-linux.git] / mm / shmem.c

/*
 * Resizable virtual memory filesystem for Linux.
 *
 * Copyright (C) 2000 Linus Torvalds.
 *               2000 Transmeta Corp.
 *               2000-2001 Christoph Rohland
 *               2000-2001 SAP AG
 *               2002 Red Hat Inc.
 * Copyright (C) 2002-2011 Hugh Dickins.
 * Copyright (C) 2011 Google Inc.
 * Copyright (C) 2002-2005 VERITAS Software Corporation.
 * Copyright (C) 2004 Andi Kleen, SuSE Labs
 *
 * Extended attribute support for tmpfs:
 * Copyright (c) 2004, Luke Kenneth Casson Leighton <lkcl@lkcl.net>
 * Copyright (c) 2004 Red Hat, Inc., James Morris <jmorris@redhat.com>
 *
 * tiny-shmem:
 * Copyright (c) 2004, 2008 Matt Mackall <mpm@selenic.com>
 *
 * This file is released under the GPL.
 */

#include <linux/fs.h>
#include <linux/init.h>
#include <linux/vfs.h>
#include <linux/mount.h>
#include <linux/ramfs.h>
#include <linux/pagemap.h>
#include <linux/file.h>
#include <linux/mm.h>
#include <linux/export.h>
#include <linux/swap.h>
#include <linux/aio.h>

static struct vfsmount *shm_mnt;

#ifdef CONFIG_SHMEM
/*
 * This virtual memory filesystem is heavily based on the ramfs. It
 * extends ramfs by the ability to use swap and honor resource limits
 * which makes it a completely usable filesystem.
 */

#include <linux/xattr.h>
#include <linux/exportfs.h>
#include <linux/posix_acl.h>
#include <linux/posix_acl_xattr.h>
#include <linux/mman.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/backing-dev.h>
#include <linux/shmem_fs.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/percpu_counter.h>
#include <linux/falloc.h>
#include <linux/splice.h>
#include <linux/security.h>
#include <linux/swapops.h>
#include <linux/mempolicy.h>
#include <linux/namei.h>
#include <linux/ctype.h>
#include <linux/migrate.h>
#include <linux/highmem.h>
#include <linux/seq_file.h>
#include <linux/magic.h>

#include <asm/uaccess.h>
#include <asm/pgtable.h>

#define BLOCKS_PER_PAGE  (PAGE_CACHE_SIZE/512)
#define VM_ACCT(size)    (PAGE_CACHE_ALIGN(size) >> PAGE_SHIFT)

/* Pretend that each entry is of this size in directory's i_size */
#define BOGO_DIRENT_SIZE 20

/* Symlink up to this size is kmalloc'ed instead of using a swappable page */
#define SHORT_SYMLINK_LEN 128

/*
 * shmem_fallocate communicates with shmem_fault or shmem_writepage via
 * inode->i_private (with i_mutex making sure that it has only one user at
 * a time): we would prefer not to enlarge the shmem inode just for that.
 */
struct shmem_falloc {
        wait_queue_head_t *waitq; /* faults into hole wait for punch to end */
        pgoff_t start;          /* start of range currently being fallocated */
        pgoff_t next;           /* the next page offset to be fallocated */
        pgoff_t nr_falloced;    /* how many new pages have been fallocated */
        pgoff_t nr_unswapped;   /* how often writepage refused to swap out */
};

/* Flag allocation requirements to shmem_getpage */
enum sgp_type {
        SGP_READ,       /* don't exceed i_size, don't allocate page */
        SGP_CACHE,      /* don't exceed i_size, may allocate page */
        SGP_DIRTY,      /* like SGP_CACHE, but set new page dirty */
        SGP_WRITE,      /* may exceed i_size, may allocate !Uptodate page */
        SGP_FALLOC,     /* like SGP_WRITE, but make existing page Uptodate */
};

#ifdef CONFIG_TMPFS
static unsigned long shmem_default_max_blocks(void)
{
        return totalram_pages / 2;
}

static unsigned long shmem_default_max_inodes(void)
{
        return min(totalram_pages - totalhigh_pages, totalram_pages / 2);
}
#endif

static bool shmem_should_replace_page(struct page *page, gfp_t gfp);
static int shmem_replace_page(struct page **pagep, gfp_t gfp,
                                struct shmem_inode_info *info, pgoff_t index);
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
        struct page **pagep, enum sgp_type sgp, gfp_t gfp, int *fault_type);

static inline int shmem_getpage(struct inode *inode, pgoff_t index,
        struct page **pagep, enum sgp_type sgp, int *fault_type)
{
        return shmem_getpage_gfp(inode, index, pagep, sgp,
                        mapping_gfp_mask(inode->i_mapping), fault_type);
}

static inline struct shmem_sb_info *SHMEM_SB(struct super_block *sb)
{
        return sb->s_fs_info;
}

/*
 * shmem_file_setup pre-accounts the whole fixed size of a VM object,
 * for shared memory and for shared anonymous (/dev/zero) mappings
 * (unless MAP_NORESERVE and sysctl_overcommit_memory <= 1),
 * consistent with the pre-accounting of private mappings ...
 */
static inline int shmem_acct_size(unsigned long flags, loff_t size)
{
        return (flags & VM_NORESERVE) ?
                0 : security_vm_enough_memory_mm(current->mm, VM_ACCT(size));
}

static inline void shmem_unacct_size(unsigned long flags, loff_t size)
{
        if (!(flags & VM_NORESERVE))
                vm_unacct_memory(VM_ACCT(size));
}

/*
 * ... whereas tmpfs objects are accounted incrementally as
 * pages are allocated, in order to allow huge sparse files.
 * shmem_getpage reports shmem_acct_block failure as -ENOSPC not -ENOMEM,
 * so that a failure on a sparse tmpfs mapping will give SIGBUS not OOM.
 */
static inline int shmem_acct_block(unsigned long flags)
{
        return (flags & VM_NORESERVE) ?
                security_vm_enough_memory_mm(current->mm, VM_ACCT(PAGE_CACHE_SIZE)) : 0;
}

static inline void shmem_unacct_blocks(unsigned long flags, long pages)
{
        if (flags & VM_NORESERVE)
                vm_unacct_memory(pages * VM_ACCT(PAGE_CACHE_SIZE));
}

static const struct super_operations shmem_ops;
static const struct address_space_operations shmem_aops;
static const struct file_operations shmem_file_operations;
static const struct inode_operations shmem_inode_operations;
static const struct inode_operations shmem_dir_inode_operations;
static const struct inode_operations shmem_special_inode_operations;
static const struct vm_operations_struct shmem_vm_ops;

static struct backing_dev_info shmem_backing_dev_info  __read_mostly = {
        .ra_pages       = 0,    /* No readahead */
        .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
};

static LIST_HEAD(shmem_swaplist);
static DEFINE_MUTEX(shmem_swaplist_mutex);

static int shmem_reserve_inode(struct super_block *sb)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
        if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                if (!sbinfo->free_inodes) {
                        spin_unlock(&sbinfo->stat_lock);
                        return -ENOSPC;
                }
                sbinfo->free_inodes--;
                spin_unlock(&sbinfo->stat_lock);
        }
        return 0;
}

static void shmem_free_inode(struct super_block *sb)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
        if (sbinfo->max_inodes) {
                spin_lock(&sbinfo->stat_lock);
                sbinfo->free_inodes++;
                spin_unlock(&sbinfo->stat_lock);
        }
}

/**
 * shmem_recalc_inode - recalculate the block usage of an inode
 * @inode: inode to recalc
 *
 * We have to calculate the free blocks since the mm can drop
 * undirtied hole pages behind our back.
 *
 * But normally   info->alloced == inode->i_mapping->nrpages + info->swapped
 * So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped)
 *
 * It has to be called with the spinlock held.
 */
static void shmem_recalc_inode(struct inode *inode)
{
        struct shmem_inode_info *info = SHMEM_I(inode);
        long freed;

        freed = info->alloced - info->swapped - inode->i_mapping->nrpages;
        if (freed > 0) {
                struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
                if (sbinfo->max_blocks)
                        percpu_counter_add(&sbinfo->used_blocks, -freed);
                info->alloced -= freed;
                inode->i_blocks -= freed * BLOCKS_PER_PAGE;
                shmem_unacct_blocks(info->flags, freed);
        }
}

/*
 * Replace item expected in radix tree by a new item, while holding tree lock.
 */
static int shmem_radix_tree_replace(struct address_space *mapping,
                        pgoff_t index, void *expected, void *replacement)
{
        void **pslot;
        void *item;

        VM_BUG_ON(!expected);
        VM_BUG_ON(!replacement);
        pslot = radix_tree_lookup_slot(&mapping->page_tree, index);
        if (!pslot)
                return -ENOENT;
        item = radix_tree_deref_slot_protected(pslot, &mapping->tree_lock);
        if (item != expected)
                return -ENOENT;
        radix_tree_replace_slot(pslot, replacement);
        return 0;
}

/*
 * Sometimes, before we decide whether to proceed or to fail, we must check
 * that an entry was not already brought back from swap by a racing thread.
 *
 * Checking page is not enough: by the time a SwapCache page is locked, it
 * might be reused, and again be SwapCache, using the same swap as before.
 */
static bool shmem_confirm_swap(struct address_space *mapping,
                               pgoff_t index, swp_entry_t swap)
{
        void *item;

        rcu_read_lock();
        item = radix_tree_lookup(&mapping->page_tree, index);
        rcu_read_unlock();
        return item == swp_to_radix_entry(swap);
}

/*
 * Like add_to_page_cache_locked, but error if expected item has gone.
 */
static int shmem_add_to_page_cache(struct page *page,
                                   struct address_space *mapping,
                                   pgoff_t index, gfp_t gfp, void *expected)
{
        int error;

        VM_BUG_ON_PAGE(!PageLocked(page), page);
        VM_BUG_ON_PAGE(!PageSwapBacked(page), page);

        page_cache_get(page);
        page->mapping = mapping;
        page->index = index;

        spin_lock_irq(&mapping->tree_lock);
        if (!expected)
                error = radix_tree_insert(&mapping->page_tree, index, page);
        else
                error = shmem_radix_tree_replace(mapping, index, expected,
                                                                 page);
        if (!error) {
                mapping->nrpages++;
                __inc_zone_page_state(page, NR_FILE_PAGES);
                __inc_zone_page_state(page, NR_SHMEM);
                spin_unlock_irq(&mapping->tree_lock);
        } else {
                page->mapping = NULL;
                spin_unlock_irq(&mapping->tree_lock);
                page_cache_release(page);
        }
        return error;
}

/*
 * Like delete_from_page_cache, but substitutes swap for page.
 */
static void shmem_delete_from_page_cache(struct page *page, void *radswap)
{
        struct address_space *mapping = page->mapping;
        int error;

        spin_lock_irq(&mapping->tree_lock);
        error = shmem_radix_tree_replace(mapping, page->index, page, radswap);
        page->mapping = NULL;
        mapping->nrpages--;
        __dec_zone_page_state(page, NR_FILE_PAGES);
        __dec_zone_page_state(page, NR_SHMEM);
        spin_unlock_irq(&mapping->tree_lock);
        page_cache_release(page);
        BUG_ON(error);
}

/*
 * Remove swap entry from radix tree, free the swap and its page cache.
 */
static int shmem_free_swap(struct address_space *mapping,
                           pgoff_t index, void *radswap)
{
        void *old;

        spin_lock_irq(&mapping->tree_lock);
        old = radix_tree_delete_item(&mapping->page_tree, index, radswap);
        spin_unlock_irq(&mapping->tree_lock);
        if (old != radswap)
                return -ENOENT;
        free_swap_and_cache(radix_to_swp_entry(radswap));
        return 0;
}

/*
 * SysV IPC SHM_UNLOCK restore Unevictable pages to their evictable lists.
 */
void shmem_unlock_mapping(struct address_space *mapping)
{
        struct pagevec pvec;
        pgoff_t indices[PAGEVEC_SIZE];
        pgoff_t index = 0;

        pagevec_init(&pvec, 0);
        /*
         * Minor point, but we might as well stop if someone else SHM_LOCKs it.
         */
        while (!mapping_unevictable(mapping)) {
                /*
                 * Avoid pagevec_lookup(): find_get_pages() returns 0 as if it
                 * has finished, if it hits a row of PAGEVEC_SIZE swap entries.
                 */
                pvec.nr = find_get_entries(mapping, index,
                                           PAGEVEC_SIZE, pvec.pages, indices);
                if (!pvec.nr)
                        break;
                index = indices[pvec.nr - 1] + 1;
                pagevec_remove_exceptionals(&pvec);
                check_move_unevictable_pages(pvec.pages, pvec.nr);
                pagevec_release(&pvec);
                cond_resched();
        }
}

/*
 * Remove range of pages and swap entries from radix tree, and free them.
 * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
 */
static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
                                                                 bool unfalloc)
{
        struct address_space *mapping = inode->i_mapping;
        struct shmem_inode_info *info = SHMEM_I(inode);
        pgoff_t start = (lstart + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
        pgoff_t end = (lend + 1) >> PAGE_CACHE_SHIFT;
        unsigned int partial_start = lstart & (PAGE_CACHE_SIZE - 1);
        unsigned int partial_end = (lend + 1) & (PAGE_CACHE_SIZE - 1);
        struct pagevec pvec;
        pgoff_t indices[PAGEVEC_SIZE];
        long nr_swaps_freed = 0;
        pgoff_t index;
        int i;

        if (lend == -1)
                end = -1;       /* unsigned, so actually very big */

        pagevec_init(&pvec, 0);
        index = start;
        while (index < end) {
                pvec.nr = find_get_entries(mapping, index,
                        min(end - index, (pgoff_t)PAGEVEC_SIZE),
                        pvec.pages, indices);
                if (!pvec.nr)
                        break;
                mem_cgroup_uncharge_start();
                for (i = 0; i < pagevec_count(&pvec); i++) {
                        struct page *page = pvec.pages[i];

                        index = indices[i];
                        if (index >= end)
                                break;

                        if (radix_tree_exceptional_entry(page)) {
                                if (unfalloc)
                                        continue;
                                nr_swaps_freed += !shmem_free_swap(mapping,
                                                                index, page);
                                continue;
                        }

                        if (!trylock_page(page))
                                continue;
                        if (!unfalloc || !PageUptodate(page)) {
                                if (page->mapping == mapping) {
                                        VM_BUG_ON_PAGE(PageWriteback(page), page);
                                        truncate_inode_page(mapping, page);
                                }
                        }
                        unlock_page(page);
                }
                pagevec_remove_exceptionals(&pvec);
                pagevec_release(&pvec);
                mem_cgroup_uncharge_end();
                cond_resched();
                index++;
        }

        if (partial_start) {
                struct page *page = NULL;
                shmem_getpage(inode, start - 1, &page, SGP_READ, NULL);
                if (page) {
                        unsigned int top = PAGE_CACHE_SIZE;
                        if (start > end) {
                                top = partial_end;
                                partial_end = 0;
                        }
                        zero_user_segment(page, partial_start, top);
                        set_page_dirty(page);
                        unlock_page(page);
                        page_cache_release(page);
                }
        }
        if (partial_end) {
                struct page *page = NULL;
                shmem_getpage(inode, end, &page, SGP_READ, NULL);
                if (page) {
                        zero_user_segment(page, 0, partial_end);
                        set_page_dirty(page);
                        unlock_page(page);
                        page_cache_release(page);
                }
        }
        if (start >= end)
                return;

        index = start;
        while (index < end) {
                cond_resched();

                pvec.nr = find_get_entries(mapping, index,
                                min(end - index, (pgoff_t)PAGEVEC_SIZE),
                                pvec.pages, indices);
                if (!pvec.nr) {
                        /* If all gone or hole-punch or unfalloc, we're done */
                        if (index == start || end != -1)
                                break;
                        /* But if truncating, restart to make sure all gone */
                        index = start;
                        continue;
                }
                mem_cgroup_uncharge_start();
                for (i = 0; i < pagevec_count(&pvec); i++) {
                        struct page *page = pvec.pages[i];

                        index = indices[i];
                        if (index >= end)
                                break;

                        if (radix_tree_exceptional_entry(page)) {
                                if (unfalloc)
                                        continue;
                                if (shmem_free_swap(mapping, index, page)) {
                                        /* Swap was replaced by page: retry */
                                        index--;
                                        break;
                                }
                                nr_swaps_freed++;
                                continue;
                        }

                        lock_page(page);
                        if (!unfalloc || !PageUptodate(page)) {
                                if (page->mapping == mapping) {
                                        VM_BUG_ON_PAGE(PageWriteback(page), page);
                                        truncate_inode_page(mapping, page);
                                } else {
                                        /* Page was replaced by swap: retry */
                                        unlock_page(page);
                                        index--;
                                        break;
                                }
                        }
                        unlock_page(page);
                }
                pagevec_remove_exceptionals(&pvec);
                pagevec_release(&pvec);
                mem_cgroup_uncharge_end();
                index++;
        }

        spin_lock(&info->lock);
        info->swapped -= nr_swaps_freed;
        shmem_recalc_inode(inode);
        spin_unlock(&info->lock);
}

void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
{
        shmem_undo_range(inode, lstart, lend, false);
        inode->i_ctime = inode->i_mtime = CURRENT_TIME;
}
EXPORT_SYMBOL_GPL(shmem_truncate_range);

static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
{
        struct inode *inode = dentry->d_inode;
        int error;

        error = inode_change_ok(inode, attr);
        if (error)
                return error;

        if (S_ISREG(inode->i_mode) && (attr->ia_valid & ATTR_SIZE)) {
                loff_t oldsize = inode->i_size;
                loff_t newsize = attr->ia_size;

                if (newsize != oldsize) {
                        i_size_write(inode, newsize);
                        inode->i_ctime = inode->i_mtime = CURRENT_TIME;
                }
                if (newsize < oldsize) {
                        loff_t holebegin = round_up(newsize, PAGE_SIZE);
                        unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
                        shmem_truncate_range(inode, newsize, (loff_t)-1);
                        /* unmap again to remove racily COWed private pages */
                        unmap_mapping_range(inode->i_mapping, holebegin, 0, 1);
                }
        }

        setattr_copy(inode, attr);
        if (attr->ia_valid & ATTR_MODE)
                error = posix_acl_chmod(inode, inode->i_mode);
        return error;
}

static void shmem_evict_inode(struct inode *inode)
{
        struct shmem_inode_info *info = SHMEM_I(inode);

        if (inode->i_mapping->a_ops == &shmem_aops) {
                shmem_unacct_size(info->flags, inode->i_size);
                inode->i_size = 0;
                shmem_truncate_range(inode, 0, (loff_t)-1);
                if (!list_empty(&info->swaplist)) {
                        mutex_lock(&shmem_swaplist_mutex);
                        list_del_init(&info->swaplist);
                        mutex_unlock(&shmem_swaplist_mutex);
                }
        } else
                kfree(info->symlink);

        simple_xattrs_free(&info->xattrs);
        WARN_ON(inode->i_blocks);
        shmem_free_inode(inode->i_sb);
        clear_inode(inode);
}

/*
 * If swap found in inode, free it and move page from swapcache to filecache.
 */
static int shmem_unuse_inode(struct shmem_inode_info *info,
                             swp_entry_t swap, struct page **pagep)
{
        struct address_space *mapping = info->vfs_inode.i_mapping;
        void *radswap;
        pgoff_t index;
        gfp_t gfp;
        int error = 0;

        radswap = swp_to_radix_entry(swap);
        index = radix_tree_locate_item(&mapping->page_tree, radswap);
        if (index == -1)
                return 0;

        /*
         * Move _head_ to start search for next from here.
         * But be careful: shmem_evict_inode checks list_empty without taking
         * mutex, and there's an instant in list_move_tail when info->swaplist
         * would appear empty, if it were the only one on shmem_swaplist.
         */
        if (shmem_swaplist.next != &info->swaplist)
                list_move_tail(&shmem_swaplist, &info->swaplist);

        gfp = mapping_gfp_mask(mapping);
        if (shmem_should_replace_page(*pagep, gfp)) {
                mutex_unlock(&shmem_swaplist_mutex);
                error = shmem_replace_page(pagep, gfp, info, index);
                mutex_lock(&shmem_swaplist_mutex);
                /*
                 * We needed to drop mutex to make that restrictive page
                 * allocation, but the inode might have been freed while we
                 * dropped it: although a racing shmem_evict_inode() cannot
                 * complete without emptying the radix_tree, our page lock
                 * on this swapcache page is not enough to prevent that -
                 * free_swap_and_cache() of our swap entry will only
                 * trylock_page(), removing swap from radix_tree whatever.
                 *
                 * We must not proceed to shmem_add_to_page_cache() if the
                 * inode has been freed, but of course we cannot rely on
                 * inode or mapping or info to check that.  However, we can
                 * safely check if our swap entry is still in use (and here
                 * it can't have got reused for another page): if it's still
                 * in use, then the inode cannot have been freed yet, and we
                 * can safely proceed (if it's no longer in use, that tells
                 * nothing about the inode, but we don't need to unuse swap).
                 */
                if (!page_swapcount(*pagep))
                        error = -ENOENT;
        }

        /*
         * We rely on shmem_swaplist_mutex, not only to protect the swaplist,
         * but also to hold up shmem_evict_inode(): so inode cannot be freed
         * beneath us (pagelock doesn't help until the page is in pagecache).
         */
        if (!error)
                error = shmem_add_to_page_cache(*pagep, mapping, index,
                                                GFP_NOWAIT, radswap);
        if (error != -ENOMEM) {
                /*
                 * Truncation and eviction use free_swap_and_cache(), which
                 * only does trylock page: if we raced, best clean up here.
                 */
                delete_from_swap_cache(*pagep);
                set_page_dirty(*pagep);
                if (!error) {
                        spin_lock(&info->lock);
                        info->swapped--;
                        spin_unlock(&info->lock);
                        swap_free(swap);
                }
                error = 1;      /* not an error, but entry was found */
        }
        return error;
}

/*
 * Search through swapped inodes to find and replace swap by page.
 */
int shmem_unuse(swp_entry_t swap, struct page *page)
{
        struct list_head *this, *next;
        struct shmem_inode_info *info;
        int found = 0;
        int error = 0;

        /*
         * There's a faint possibility that swap page was replaced before
         * caller locked it: caller will come back later with the right page.
         */
        if (unlikely(!PageSwapCache(page) || page_private(page) != swap.val))
                goto out;

        /*
         * Charge page using GFP_KERNEL while we can wait, before taking
         * the shmem_swaplist_mutex which might hold up shmem_writepage().
         * Charged back to the user (not to caller) when swap account is used.
         */
        error = mem_cgroup_charge_file(page, current->mm, GFP_KERNEL);
        if (error)
                goto out;
        /* No radix_tree_preload: swap entry keeps a place for page in tree */

        mutex_lock(&shmem_swaplist_mutex);
        list_for_each_safe(this, next, &shmem_swaplist) {
                info = list_entry(this, struct shmem_inode_info, swaplist);
                if (info->swapped)
                        found = shmem_unuse_inode(info, swap, &page);
                else
                        list_del_init(&info->swaplist);
                cond_resched();
                if (found)
                        break;
        }
        mutex_unlock(&shmem_swaplist_mutex);

        if (found < 0)
                error = found;
out:
        unlock_page(page);
        page_cache_release(page);
        return error;
}

/*
 * Move the page from the page cache to the swap cache.
 */
static int shmem_writepage(struct page *page, struct writeback_control *wbc)
{
        struct shmem_inode_info *info;
        struct address_space *mapping;
        struct inode *inode;
        swp_entry_t swap;
        pgoff_t index;

        BUG_ON(!PageLocked(page));
        mapping = page->mapping;
        index = page->index;
        inode = mapping->host;
        info = SHMEM_I(inode);
        if (info->flags & VM_LOCKED)
                goto redirty;
        if (!total_swap_pages)
                goto redirty;

        /*
         * shmem_backing_dev_info's capabilities prevent regular writeback or
         * sync from ever calling shmem_writepage; but a stacking filesystem
         * might use ->writepage of its underlying filesystem, in which case
         * tmpfs should write out to swap only in response to memory pressure,
         * and not for the writeback threads or sync.
         */
        if (!wbc->for_reclaim) {
                WARN_ON_ONCE(1);        /* Still happens? Tell us about it! */
                goto redirty;
        }

        /*
         * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
         * value into swapfile.c, the only way we can correctly account for a
         * fallocated page arriving here is now to initialize it and write it.
         *
         * That's okay for a page already fallocated earlier, but if we have
         * not yet completed the fallocation, then (a) we want to keep track
         * of this page in case we have to undo it, and (b) it may not be a
         * good idea to continue anyway, once we're pushing into swap.  So
         * reactivate the page, and let shmem_fallocate() quit when too many.
         */
        if (!PageUptodate(page)) {
                if (inode->i_private) {
                        struct shmem_falloc *shmem_falloc;
                        spin_lock(&inode->i_lock);
                        shmem_falloc = inode->i_private;
                        if (shmem_falloc &&
                            !shmem_falloc->waitq &&
                            index >= shmem_falloc->start &&
                            index < shmem_falloc->next)
                                shmem_falloc->nr_unswapped++;
                        else
                                shmem_falloc = NULL;
                        spin_unlock(&inode->i_lock);
                        if (shmem_falloc)
                                goto redirty;
                }
                clear_highpage(page);
                flush_dcache_page(page);
                SetPageUptodate(page);
        }

        swap = get_swap_page();
        if (!swap.val)
                goto redirty;

        /*
         * Add inode to shmem_unuse()'s list of swapped-out inodes,
         * if it's not already there.  Do it now before the page is
         * moved to swap cache, when its pagelock no longer protects
         * the inode from eviction.  But don't unlock the mutex until
         * we've incremented swapped, because shmem_unuse_inode() will
         * prune a !swapped inode from the swaplist under this mutex.
         */
        mutex_lock(&shmem_swaplist_mutex);
        if (list_empty(&info->swaplist))
                list_add_tail(&info->swaplist, &shmem_swaplist);

        if (add_to_swap_cache(page, swap, GFP_ATOMIC) == 0) {
                swap_shmem_alloc(swap);
                shmem_delete_from_page_cache(page, swp_to_radix_entry(swap));

                spin_lock(&info->lock);
                info->swapped++;
                shmem_recalc_inode(inode);
                spin_unlock(&info->lock);

                mutex_unlock(&shmem_swaplist_mutex);
                BUG_ON(page_mapped(page));
                swap_writepage(page, wbc);
                return 0;
        }

        mutex_unlock(&shmem_swaplist_mutex);
        swapcache_free(swap, NULL);
redirty:
        set_page_dirty(page);
        if (wbc->for_reclaim)
                return AOP_WRITEPAGE_ACTIVATE;  /* Return with page locked */
        unlock_page(page);
        return 0;
}

#ifdef CONFIG_NUMA
#ifdef CONFIG_TMPFS
static void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
{
        char buffer[64];

        if (!mpol || mpol->mode == MPOL_DEFAULT)
                return;         /* show nothing */

        mpol_to_str(buffer, sizeof(buffer), mpol);

        seq_printf(seq, ",mpol=%s", buffer);
}

static struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
{
        struct mempolicy *mpol = NULL;
        if (sbinfo->mpol) {
                spin_lock(&sbinfo->stat_lock);  /* prevent replace/use races */
                mpol = sbinfo->mpol;
                mpol_get(mpol);
                spin_unlock(&sbinfo->stat_lock);
        }
        return mpol;
}
#endif /* CONFIG_TMPFS */

static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
                        struct shmem_inode_info *info, pgoff_t index)
{
        struct vm_area_struct pvma;
        struct page *page;

        /* Create a pseudo vma that just contains the policy */
        pvma.vm_start = 0;
        /* Bias interleave by inode number to distribute better across nodes */
        pvma.vm_pgoff = index + info->vfs_inode.i_ino;
        pvma.vm_ops = NULL;
        pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, index);

        page = swapin_readahead(swap, gfp, &pvma, 0);

        /* Drop reference taken by mpol_shared_policy_lookup() */
        mpol_cond_put(pvma.vm_policy);

        return page;
}

static struct page *shmem_alloc_page(gfp_t gfp,
                        struct shmem_inode_info *info, pgoff_t index)
{
        struct vm_area_struct pvma;
        struct page *page;

        /* Create a pseudo vma that just contains the policy */
        pvma.vm_start = 0;
        /* Bias interleave by inode number to distribute better across nodes */
        pvma.vm_pgoff = index + info->vfs_inode.i_ino;
        pvma.vm_ops = NULL;
        pvma.vm_policy = mpol_shared_policy_lookup(&info->policy, index);

        page = alloc_page_vma(gfp, &pvma, 0);

        /* Drop reference taken by mpol_shared_policy_lookup() */
        mpol_cond_put(pvma.vm_policy);

        return page;
}
#else /* !CONFIG_NUMA */
#ifdef CONFIG_TMPFS
static inline void shmem_show_mpol(struct seq_file *seq, struct mempolicy *mpol)
{
}
#endif /* CONFIG_TMPFS */

static inline struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
                        struct shmem_inode_info *info, pgoff_t index)
{
        return swapin_readahead(swap, gfp, NULL, 0);
}

static inline struct page *shmem_alloc_page(gfp_t gfp,
                        struct shmem_inode_info *info, pgoff_t index)
{
        return alloc_page(gfp);
}
#endif /* CONFIG_NUMA */

#if !defined(CONFIG_NUMA) || !defined(CONFIG_TMPFS)
static inline struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
{
        return NULL;
}
#endif

/*
 * When a page is moved from swapcache to shmem filecache (either by the
 * usual swapin of shmem_getpage_gfp(), or by the less common swapoff of
 * shmem_unuse_inode()), it may have been read in earlier from swap, in
 * ignorance of the mapping it belongs to.  If that mapping has special
 * constraints (like the gma500 GEM driver, which requires RAM below 4GB),
 * we may need to copy to a suitable page before moving to filecache.
 *
 * In a future release, this may well be extended to respect cpuset and
 * NUMA mempolicy, and applied also to anonymous pages in do_swap_page();
 * but for now it is a simple matter of zone.
 */
static bool shmem_should_replace_page(struct page *page, gfp_t gfp)
{
        return page_zonenum(page) > gfp_zone(gfp);
}

static int shmem_replace_page(struct page **pagep, gfp_t gfp,
                                struct shmem_inode_info *info, pgoff_t index)
{
        struct page *oldpage, *newpage;
        struct address_space *swap_mapping;
        pgoff_t swap_index;
        int error;

        oldpage = *pagep;
        swap_index = page_private(oldpage);
        swap_mapping = page_mapping(oldpage);

        /*
         * We have arrived here because our zones are constrained, so don't
         * limit chance of success by further cpuset and node constraints.
         */
        gfp &= ~GFP_CONSTRAINT_MASK;
        newpage = shmem_alloc_page(gfp, info, index);
        if (!newpage)
                return -ENOMEM;

        page_cache_get(newpage);
        copy_highpage(newpage, oldpage);
        flush_dcache_page(newpage);

        __set_page_locked(newpage);
        SetPageUptodate(newpage);
        SetPageSwapBacked(newpage);
        set_page_private(newpage, swap_index);
        SetPageSwapCache(newpage);

        /*
         * Our caller will very soon move newpage out of swapcache, but it's
         * a nice clean interface for us to replace oldpage by newpage there.
         */
        spin_lock_irq(&swap_mapping->tree_lock);
        error = shmem_radix_tree_replace(swap_mapping, swap_index, oldpage,
                                                                   newpage);
        if (!error) {
                __inc_zone_page_state(newpage, NR_FILE_PAGES);
                __dec_zone_page_state(oldpage, NR_FILE_PAGES);
        }
        spin_unlock_irq(&swap_mapping->tree_lock);

        if (unlikely(error)) {
                /*
                 * Is this possible?  I think not, now that our callers check
                 * both PageSwapCache and page_private after getting page lock;
                 * but be defensive.  Reverse old to newpage for clear and free.
                 */
                oldpage = newpage;
        } else {
                mem_cgroup_replace_page_cache(oldpage, newpage);
                lru_cache_add_anon(newpage);
                *pagep = newpage;
        }

        ClearPageSwapCache(oldpage);
        set_page_private(oldpage, 0);

        unlock_page(oldpage);
        page_cache_release(oldpage);
        page_cache_release(oldpage);
        return error;
}

/*
 * shmem_getpage_gfp - find page in cache, or get from swap, or allocate
 *
 * If we allocate a new one we do not mark it dirty. That's up to the
 * vm. If we swap it in we mark it dirty since we also free the swap
 * entry since a page cannot live in both the swap and page cache
 */
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
        struct page **pagep, enum sgp_type sgp, gfp_t gfp, int *fault_type)
{
        struct address_space *mapping = inode->i_mapping;
        struct shmem_inode_info *info;
        struct shmem_sb_info *sbinfo;
        struct page *page;
        swp_entry_t swap;
        int error;
        int once = 0;
        int alloced = 0;

        if (index > (MAX_LFS_FILESIZE >> PAGE_CACHE_SHIFT))
                return -EFBIG;
repeat:
        swap.val = 0;
        page = find_lock_entry(mapping, index);
        if (radix_tree_exceptional_entry(page)) {
                swap = radix_to_swp_entry(page);
                page = NULL;
        }

        if (sgp != SGP_WRITE && sgp != SGP_FALLOC &&
            ((loff_t)index << PAGE_CACHE_SHIFT) >= i_size_read(inode)) {
                error = -EINVAL;
                goto failed;
        }

        if (page && sgp == SGP_WRITE)
                mark_page_accessed(page);

        /* fallocated page? */
        if (page && !PageUptodate(page)) {
                if (sgp != SGP_READ)
                        goto clear;
                unlock_page(page);
                page_cache_release(page);
                page = NULL;
        }
        if (page || (sgp == SGP_READ && !swap.val)) {
                *pagep = page;
                return 0;
        }

        /*
         * Fast cache lookup did not find it:
         * bring it back from swap or allocate.
         */
        info = SHMEM_I(inode);
        sbinfo = SHMEM_SB(inode->i_sb);

        if (swap.val) {
                /* Look it up and read it in.. */
                page = lookup_swap_cache(swap);
                if (!page) {
                        /* here we actually do the io */
                        if (fault_type)
                                *fault_type |= VM_FAULT_MAJOR;
                        page = shmem_swapin(swap, gfp, info, index);
                        if (!page) {
                                error = -ENOMEM;
                                goto failed;
                        }
                }

                /* We have to do this with page locked to prevent races */
                lock_page(page);
                if (!PageSwapCache(page) || page_private(page) != swap.val ||
                    !shmem_confirm_swap(mapping, index, swap)) {
                        error = -EEXIST;        /* try again */
                        goto unlock;
                }
                if (!PageUptodate(page)) {
                        error = -EIO;
                        goto failed;
                }
                wait_on_page_writeback(page);

                if (shmem_should_replace_page(page, gfp)) {
                        error = shmem_replace_page(&page, gfp, info, index);
                        if (error)
                                goto failed;
                }

                error = mem_cgroup_charge_file(page, current->mm,
                                                gfp & GFP_RECLAIM_MASK);
                if (!error) {
                        error = shmem_add_to_page_cache(page, mapping, index,
                                                gfp, swp_to_radix_entry(swap));
                        /*
                         * We already confirmed swap under page lock, and make
                         * no memory allocation here, so usually no possibility
                         * of error; but free_swap_and_cache() only trylocks a
                         * page, so it is just possible that the entry has been
                         * truncated or holepunched since swap was confirmed.
                         * shmem_undo_range() will have done some of the
                         * unaccounting, now delete_from_swap_cache() will do
                         * the rest (including mem_cgroup_uncharge_swapcache).
                         * Reset swap.val? No, leave it so "failed" goes back to
                         * "repeat": reading a hole and writing should succeed.
                         */
                        if (error)
                                delete_from_swap_cache(page);
                }
                if (error)
                        goto failed;

                spin_lock(&info->lock);
                info->swapped--;
                shmem_recalc_inode(inode);
                spin_unlock(&info->lock);

                if (sgp == SGP_WRITE)
                        mark_page_accessed(page);

                delete_from_swap_cache(page);
                set_page_dirty(page);
                swap_free(swap);

        } else {
                if (shmem_acct_block(info->flags)) {
                        error = -ENOSPC;
                        goto failed;
                }
                if (sbinfo->max_blocks) {
                        if (percpu_counter_compare(&sbinfo->used_blocks,
                                                sbinfo->max_blocks) >= 0) {
                                error = -ENOSPC;
                                goto unacct;
                        }
                        percpu_counter_inc(&sbinfo->used_blocks);
                }

                page = shmem_alloc_page(gfp, info, index);
                if (!page) {
                        error = -ENOMEM;
                        goto decused;
                }

                __SetPageSwapBacked(page);
                __set_page_locked(page);
                if (sgp == SGP_WRITE)
                        init_page_accessed(page);

                error = mem_cgroup_charge_file(page, current->mm,
                                                gfp & GFP_RECLAIM_MASK);
                if (error)
                        goto decused;
                error = radix_tree_maybe_preload(gfp & GFP_RECLAIM_MASK);
                if (!error) {
                        error = shmem_add_to_page_cache(page, mapping, index,
                                                        gfp, NULL);
                        radix_tree_preload_end();
                }
                if (error) {
                        mem_cgroup_uncharge_cache_page(page);
                        goto decused;
                }
                lru_cache_add_anon(page);

                spin_lock(&info->lock);
                info->alloced++;
                inode->i_blocks += BLOCKS_PER_PAGE;
                shmem_recalc_inode(inode);
                spin_unlock(&info->lock);
                alloced = true;

                /*
                 * Let SGP_FALLOC use the SGP_WRITE optimization on a new page.
                 */
                if (sgp == SGP_FALLOC)
                        sgp = SGP_WRITE;
clear:
                /*
                 * Let SGP_WRITE caller clear ends if write does not fill page;
                 * but SGP_FALLOC on a page fallocated earlier must initialize
                 * it now, lest undo on failure cancel our earlier guarantee.
                 */
                if (sgp != SGP_WRITE) {
                        clear_highpage(page);
                        flush_dcache_page(page);
                        SetPageUptodate(page);
                }
                if (sgp == SGP_DIRTY)
                        set_page_dirty(page);
        }

        /* Perhaps the file has been truncated since we checked */
        if (sgp != SGP_WRITE && sgp != SGP_FALLOC &&
            ((loff_t)index << PAGE_CACHE_SHIFT) >= i_size_read(inode)) {
                error = -EINVAL;
                if (alloced)
                        goto trunc;
                else
                        goto failed;
        }
        *pagep = page;
        return 0;

        /*
         * Error recovery.
         */
trunc:
        info = SHMEM_I(inode);
        ClearPageDirty(page);
        delete_from_page_cache(page);
        spin_lock(&info->lock);
        info->alloced--;
        inode->i_blocks -= BLOCKS_PER_PAGE;
        spin_unlock(&info->lock);
decused:
        sbinfo = SHMEM_SB(inode->i_sb);
        if (sbinfo->max_blocks)
                percpu_counter_add(&sbinfo->used_blocks, -1);
unacct:
        shmem_unacct_blocks(info->flags, 1);
failed:
        if (swap.val && error != -EINVAL &&
            !shmem_confirm_swap(mapping, index, swap))
                error = -EEXIST;
unlock:
        if (page) {
                unlock_page(page);
                page_cache_release(page);
        }
        if (error == -ENOSPC && !once++) {
                info = SHMEM_I(inode);
                spin_lock(&info->lock);
                shmem_recalc_inode(inode);
                spin_unlock(&info->lock);
                goto repeat;
        }
        if (error == -EEXIST)   /* from above or from radix_tree_insert */
                goto repeat;
        return error;
}

static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
{
        struct inode *inode = file_inode(vma->vm_file);
        int error;
        int ret = VM_FAULT_LOCKED;

        /*
         * Trinity finds that probing a hole which tmpfs is punching can
         * prevent the hole-punch from ever completing: which in turn
         * locks writers out with its hold on i_mutex.  So refrain from
         * faulting pages into the hole while it's being punched.  Although
         * shmem_undo_range() does remove the additions, it may be unable to
         * keep up, as each new page needs its own unmap_mapping_range() call,
         * and the i_mmap tree grows ever slower to scan if new vmas are added.
         *
         * It does not matter if we sometimes reach this check just before the
         * hole-punch begins, so that one fault then races with the punch:
         * we just need to make racing faults a rare case.
         *
         * The implementation below would be much simpler if we just used a
         * standard mutex or completion: but we cannot take i_mutex in fault,
         * and bloating every shmem inode for this unlikely case would be sad.
         */
        if (unlikely(inode->i_private)) {
                struct shmem_falloc *shmem_falloc;

                spin_lock(&inode->i_lock);
                shmem_falloc = inode->i_private;
                if (shmem_falloc &&
                    shmem_falloc->waitq &&
                    vmf->pgoff >= shmem_falloc->start &&
                    vmf->pgoff < shmem_falloc->next) {
                        wait_queue_head_t *shmem_falloc_waitq;
                        DEFINE_WAIT(shmem_fault_wait);

                        ret = VM_FAULT_NOPAGE;
                        if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
                           !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
                                /* It's polite to up mmap_sem if we can */
                                up_read(&vma->vm_mm->mmap_sem);
                                ret = VM_FAULT_RETRY;
                        }

                        shmem_falloc_waitq = shmem_falloc->waitq;
                        prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
                                        TASK_UNINTERRUPTIBLE);
                        spin_unlock(&inode->i_lock);
                        schedule();

                        /*
                         * shmem_falloc_waitq points into the shmem_fallocate()
                         * stack of the hole-punching task: shmem_falloc_waitq
                         * is usually invalid by the time we reach here, but
                         * finish_wait() does not dereference it in that case;
                         * though i_lock needed lest racing with wake_up_all().
                         */
                        spin_lock(&inode->i_lock);
                        finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
                        spin_unlock(&inode->i_lock);
                        return ret;
                }
                spin_unlock(&inode->i_lock);
        }

        error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
        if (error)
                return ((error == -ENOMEM) ? VM_FAULT_OOM : VM_FAULT_SIGBUS);

        if (ret & VM_FAULT_MAJOR) {
                count_vm_event(PGMAJFAULT);
                mem_cgroup_count_vm_event(vma->vm_mm, PGMAJFAULT);
        }
        return ret;
}

#ifdef CONFIG_NUMA
static int shmem_set_policy(struct vm_area_struct *vma, struct mempolicy *mpol)
{
        struct inode *inode = file_inode(vma->vm_file);
        return mpol_set_shared_policy(&SHMEM_I(inode)->policy, vma, mpol);
}

static struct mempolicy *shmem_get_policy(struct vm_area_struct *vma,
                                          unsigned long addr)
{
        struct inode *inode = file_inode(vma->vm_file);
        pgoff_t index;

        index = ((addr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
        return mpol_shared_policy_lookup(&SHMEM_I(inode)->policy, index);
}
#endif

int shmem_lock(struct file *file, int lock, struct user_struct *user)
{
        struct inode *inode = file_inode(file);
        struct shmem_inode_info *info = SHMEM_I(inode);
        int retval = -ENOMEM;

        spin_lock(&info->lock);
        if (lock && !(info->flags & VM_LOCKED)) {
                if (!user_shm_lock(inode->i_size, user))
                        goto out_nomem;
                info->flags |= VM_LOCKED;
                mapping_set_unevictable(file->f_mapping);
        }
        if (!lock && (info->flags & VM_LOCKED) && user) {
                user_shm_unlock(inode->i_size, user);
                info->flags &= ~VM_LOCKED;
                mapping_clear_unevictable(file->f_mapping);
        }
        retval = 0;

out_nomem:
        spin_unlock(&info->lock);
        return retval;
}

static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
{
        file_accessed(file);
        vma->vm_ops = &shmem_vm_ops;
        return 0;
}

static struct inode *shmem_get_inode(struct super_block *sb, const struct inode *dir,
                                     umode_t mode, dev_t dev, unsigned long flags)
{
        struct inode *inode;
        struct shmem_inode_info *info;
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

        if (shmem_reserve_inode(sb))
                return NULL;

        inode = new_inode(sb);
        if (inode) {
                inode->i_ino = get_next_ino();
                inode_init_owner(inode, dir, mode);
                inode->i_blocks = 0;
                inode->i_mapping->backing_dev_info = &shmem_backing_dev_info;
                inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
                inode->i_generation = get_seconds();
                info = SHMEM_I(inode);
                memset(info, 0, (char *)inode - (char *)info);
                spin_lock_init(&info->lock);
                info->flags = flags & VM_NORESERVE;
                INIT_LIST_HEAD(&info->swaplist);
                simple_xattrs_init(&info->xattrs);
                cache_no_acl(inode);

                switch (mode & S_IFMT) {
                default:
                        inode->i_op = &shmem_special_inode_operations;
                        init_special_inode(inode, mode, dev);
                        break;
                case S_IFREG:
                        inode->i_mapping->a_ops = &shmem_aops;
                        inode->i_op = &shmem_inode_operations;
                        inode->i_fop = &shmem_file_operations;
                        mpol_shared_policy_init(&info->policy,
                                                 shmem_get_sbmpol(sbinfo));
                        break;
                case S_IFDIR:
                        inc_nlink(inode);
                        /* Some things misbehave if size == 0 on a directory */
                        inode->i_size = 2 * BOGO_DIRENT_SIZE;
                        inode->i_op = &shmem_dir_inode_operations;
                        inode->i_fop = &simple_dir_operations;
                        break;
                case S_IFLNK:
                        /*
                         * Must not load anything in the rbtree,
                         * mpol_free_shared_policy will not be called.
                         */
                        mpol_shared_policy_init(&info->policy, NULL);
                        break;
                }
        } else
                shmem_free_inode(sb);
        return inode;
}

bool shmem_mapping(struct address_space *mapping)
{
        return mapping->backing_dev_info == &shmem_backing_dev_info;
}

#ifdef CONFIG_TMPFS
static const struct inode_operations shmem_symlink_inode_operations;
static const struct inode_operations shmem_short_symlink_operations;

#ifdef CONFIG_TMPFS_XATTR
static int shmem_initxattrs(struct inode *, const struct xattr *, void *);
#else
#define shmem_initxattrs NULL
#endif

static int
shmem_write_begin(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned flags,
                        struct page **pagep, void **fsdata)
{
        struct inode *inode = mapping->host;
        pgoff_t index = pos >> PAGE_CACHE_SHIFT;
        return shmem_getpage(inode, index, pagep, SGP_WRITE, NULL);
}

static int
shmem_write_end(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned copied,
                        struct page *page, void *fsdata)
{
        struct inode *inode = mapping->host;

        if (pos + copied > inode->i_size)
                i_size_write(inode, pos + copied);

        if (!PageUptodate(page)) {
                if (copied < PAGE_CACHE_SIZE) {
                        unsigned from = pos & (PAGE_CACHE_SIZE - 1);
                        zero_user_segments(page, 0, from,
                                        from + copied, PAGE_CACHE_SIZE);
                }
                SetPageUptodate(page);
        }
        set_page_dirty(page);
        unlock_page(page);
        page_cache_release(page);

        return copied;
}

static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
{
        struct file *file = iocb->ki_filp;
        struct inode *inode = file_inode(file);
        struct address_space *mapping = inode->i_mapping;
        pgoff_t index;
        unsigned long offset;
        enum sgp_type sgp = SGP_READ;
        int error = 0;
        ssize_t retval = 0;
        loff_t *ppos = &iocb->ki_pos;

        /*
         * Might this read be for a stacking filesystem?  Then when reading
         * holes of a sparse file, we actually need to allocate those pages,
         * and even mark them dirty, so it cannot exceed the max_blocks limit.
         */
        if (segment_eq(get_fs(), KERNEL_DS))
                sgp = SGP_DIRTY;

        index = *ppos >> PAGE_CACHE_SHIFT;
        offset = *ppos & ~PAGE_CACHE_MASK;

        for (;;) {
                struct page *page = NULL;
                pgoff_t end_index;
                unsigned long nr, ret;
                loff_t i_size = i_size_read(inode);

                end_index = i_size >> PAGE_CACHE_SHIFT;
                if (index > end_index)
                        break;
                if (index == end_index) {
                        nr = i_size & ~PAGE_CACHE_MASK;
                        if (nr <= offset)
                                break;
                }

                error = shmem_getpage(inode, index, &page, sgp, NULL);
                if (error) {
                        if (error == -EINVAL)
                                error = 0;
                        break;
                }
                if (page)
                        unlock_page(page);

                /*
                 * We must evaluate after, since reads (unlike writes)
                 * are called without i_mutex protection against truncate
                 */
                nr = PAGE_CACHE_SIZE;
                i_size = i_size_read(inode);
                end_index = i_size >> PAGE_CACHE_SHIFT;
                if (index == end_index) {
                        nr = i_size & ~PAGE_CACHE_MASK;
                        if (nr <= offset) {
                                if (page)
                                        page_cache_release(page);
                                break;
                        }
                }
                nr -= offset;

                if (page) {
                        /*
                         * If users can be writing to this page using arbitrary
                         * virtual addresses, take care about potential aliasing
                         * before reading the page on the kernel side.
                         */
                        if (mapping_writably_mapped(mapping))
                                flush_dcache_page(page);
                        /*
                         * Mark the page accessed if we read the beginning.
                         */
                        if (!offset)
                                mark_page_accessed(page);
                } else {
                        page = ZERO_PAGE(0);
                        page_cache_get(page);
                }

                /*
                 * Ok, we have the page, and it's up-to-date, so
                 * now we can copy it to user space...
                 */
                ret = copy_page_to_iter(page, offset, nr, to);
                retval += ret;
                offset += ret;
                index += offset >> PAGE_CACHE_SHIFT;
                offset &= ~PAGE_CACHE_MASK;

                page_cache_release(page);
                if (!iov_iter_count(to))
                        break;
                if (ret < nr) {
                        error = -EFAULT;
                        break;
                }
                cond_resched();
        }

        *ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
        file_accessed(file);
        return retval ? retval : error;
}

static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
                                struct pipe_inode_info *pipe, size_t len,
                                unsigned int flags)
{
        struct address_space *mapping = in->f_mapping;
        struct inode *inode = mapping->host;
        unsigned int loff, nr_pages, req_pages;
        struct page *pages[PIPE_DEF_BUFFERS];
        struct partial_page partial[PIPE_DEF_BUFFERS];
        struct page *page;
        pgoff_t index, end_index;
        loff_t isize, left;
        int error, page_nr;
        struct splice_pipe_desc spd = {
                .pages = pages,
                .partial = partial,
                .nr_pages_max = PIPE_DEF_BUFFERS,
                .flags = flags,
                .ops = &page_cache_pipe_buf_ops,
                .spd_release = spd_release_page,
        };

        isize = i_size_read(inode);
        if (unlikely(*ppos >= isize))
                return 0;

        left = isize - *ppos;
        if (unlikely(left < len))
                len = left;

        if (splice_grow_spd(pipe, &spd))
                return -ENOMEM;

        index = *ppos >> PAGE_CACHE_SHIFT;
        loff = *ppos & ~PAGE_CACHE_MASK;
        req_pages = (len + loff + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
        nr_pages = min(req_pages, spd.nr_pages_max);

        spd.nr_pages = find_get_pages_contig(mapping, index,
                                                nr_pages, spd.pages);
        index += spd.nr_pages;
        error = 0;

        while (spd.nr_pages < nr_pages) {
                error = shmem_getpage(inode, index, &page, SGP_CACHE, NULL);
                if (error)
                        break;
                unlock_page(page);
                spd.pages[spd.nr_pages++] = page;
                index++;
        }

        index = *ppos >> PAGE_CACHE_SHIFT;
        nr_pages = spd.nr_pages;
        spd.nr_pages = 0;

        for (page_nr = 0; page_nr < nr_pages; page_nr++) {
                unsigned int this_len;

                if (!len)
                        break;

                this_len = min_t(unsigned long, len, PAGE_CACHE_SIZE - loff);
                page = spd.pages[page_nr];

                if (!PageUptodate(page) || page->mapping != mapping) {
                        error = shmem_getpage(inode, index, &page,
                                                        SGP_CACHE, NULL);
                        if (error)
                                break;
                        unlock_page(page);
                        page_cache_release(spd.pages[page_nr]);
                        spd.pages[page_nr] = page;
                }

                isize = i_size_read(inode);
                end_index = (isize - 1) >> PAGE_CACHE_SHIFT;
                if (unlikely(!isize || index > end_index))
                        break;

                if (end_index == index) {
                        unsigned int plen;

                        plen = ((isize - 1) & ~PAGE_CACHE_MASK) + 1;
                        if (plen <= loff)
                                break;

                        this_len = min(this_len, plen - loff);
                        len = this_len;
                }

                spd.partial[page_nr].offset = loff;
                spd.partial[page_nr].len = this_len;
                len -= this_len;
                loff = 0;
                spd.nr_pages++;
                index++;
        }

        while (page_nr < nr_pages)
                page_cache_release(spd.pages[page_nr++]);

        if (spd.nr_pages)
                error = splice_to_pipe(pipe, &spd);

        splice_shrink_spd(&spd);

        if (error > 0) {
                *ppos += error;
                file_accessed(in);
        }
        return error;
}

/*
 * llseek SEEK_DATA or SEEK_HOLE through the radix_tree.
 */
static pgoff_t shmem_seek_hole_data(struct address_space *mapping,
                                    pgoff_t index, pgoff_t end, int whence)
{
        struct page *page;
        struct pagevec pvec;
        pgoff_t indices[PAGEVEC_SIZE];
        bool done = false;
        int i;

        pagevec_init(&pvec, 0);
        pvec.nr = 1;            /* start small: we may be there already */
        while (!done) {
                pvec.nr = find_get_entries(mapping, index,
                                        pvec.nr, pvec.pages, indices);
                if (!pvec.nr) {
                        if (whence == SEEK_DATA)
                                index = end;
                        break;
                }
                for (i = 0; i < pvec.nr; i++, index++) {
                        if (index < indices[i]) {
                                if (whence == SEEK_HOLE) {
                                        done = true;
                                        break;
                                }
                                index = indices[i];
                        }
                        page = pvec.pages[i];
                        if (page && !radix_tree_exceptional_entry(page)) {
                                if (!PageUptodate(page))
                                        page = NULL;
                        }
                        if (index >= end ||
                            (page && whence == SEEK_DATA) ||
                            (!page && whence == SEEK_HOLE)) {
                                done = true;
                                break;
                        }
                }
                pagevec_remove_exceptionals(&pvec);
                pagevec_release(&pvec);
                pvec.nr = PAGEVEC_SIZE;
                cond_resched();
        }
        return index;
}

static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
{
        struct address_space *mapping = file->f_mapping;
        struct inode *inode = mapping->host;
        pgoff_t start, end;
        loff_t new_offset;

        if (whence != SEEK_DATA && whence != SEEK_HOLE)
                return generic_file_llseek_size(file, offset, whence,
                                        MAX_LFS_FILESIZE, i_size_read(inode));
        mutex_lock(&inode->i_mutex);
        /* We're holding i_mutex so we can access i_size directly */

        if (offset < 0)
                offset = -EINVAL;
        else if (offset >= inode->i_size)
                offset = -ENXIO;
        else {
                start = offset >> PAGE_CACHE_SHIFT;
                end = (inode->i_size + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
                new_offset = shmem_seek_hole_data(mapping, start, end, whence);
                new_offset <<= PAGE_CACHE_SHIFT;
                if (new_offset > offset) {
                        if (new_offset < inode->i_size)
                                offset = new_offset;
                        else if (whence == SEEK_DATA)
                                offset = -ENXIO;
                        else
                                offset = inode->i_size;
                }
        }

        if (offset >= 0)
                offset = vfs_setpos(file, offset, MAX_LFS_FILESIZE);
        mutex_unlock(&inode->i_mutex);
        return offset;
}

static long shmem_fallocate(struct file *file, int mode, loff_t offset,
                                                         loff_t len)
{
        struct inode *inode = file_inode(file);
        struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
        struct shmem_falloc shmem_falloc;
        pgoff_t start, index, end;
        int error;

        if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
                return -EOPNOTSUPP;

        mutex_lock(&inode->i_mutex);

        if (mode & FALLOC_FL_PUNCH_HOLE) {
                struct address_space *mapping = file->f_mapping;
                loff_t unmap_start = round_up(offset, PAGE_SIZE);
                loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
                DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);

                shmem_falloc.waitq = &shmem_falloc_waitq;
                shmem_falloc.start = unmap_start >> PAGE_SHIFT;
                shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
                spin_lock(&inode->i_lock);
                inode->i_private = &shmem_falloc;
                spin_unlock(&inode->i_lock);

                if ((u64)unmap_end > (u64)unmap_start)
                        unmap_mapping_range(mapping, unmap_start,
                                            1 + unmap_end - unmap_start, 0);
                shmem_truncate_range(inode, offset, offset + len - 1);
                /* No need to unmap again: hole-punching leaves COWed pages */

                spin_lock(&inode->i_lock);
                inode->i_private = NULL;
                wake_up_all(&shmem_falloc_waitq);
                spin_unlock(&inode->i_lock);
                error = 0;
                goto out;
        }

        /* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
        error = inode_newsize_ok(inode, offset + len);
        if (error)
                goto out;

        start = offset >> PAGE_CACHE_SHIFT;
        end = (offset + len + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
        /* Try to avoid a swapstorm if len is impossible to satisfy */
        if (sbinfo->max_blocks && end - start > sbinfo->max_blocks) {
                error = -ENOSPC;
                goto out;
        }

        shmem_falloc.waitq = NULL;
        shmem_falloc.start = start;
        shmem_falloc.next  = start;
        shmem_falloc.nr_falloced = 0;
        shmem_falloc.nr_unswapped = 0;
        spin_lock(&inode->i_lock);
        inode->i_private = &shmem_falloc;
        spin_unlock(&inode->i_lock);

        for (index = start; index < end; index++) {
                struct page *page;

                /*
                 * Good, the fallocate(2) manpage permits EINTR: we may have
                 * been interrupted because we are using up too much memory.
                 */
                if (signal_pending(current))
                        error = -EINTR;
                else if (shmem_falloc.nr_unswapped > shmem_falloc.nr_falloced)
                        error = -ENOMEM;
                else
                        error = shmem_getpage(inode, index, &page, SGP_FALLOC,
                                                                        NULL);
                if (error) {
                        /* Remove the !PageUptodate pages we added */
                        shmem_undo_range(inode,
                                (loff_t)start << PAGE_CACHE_SHIFT,
                                (loff_t)index << PAGE_CACHE_SHIFT, true);
                        goto undone;
                }

                /*
                 * Inform shmem_writepage() how far we have reached.
                 * No need for lock or barrier: we have the page lock.
                 */
                shmem_falloc.next++;
                if (!PageUptodate(page))
                        shmem_falloc.nr_falloced++;

                /*
                 * If !PageUptodate, leave it that way so that freeable pages
                 * can be recognized if we need to rollback on error later.
                 * But set_page_dirty so that memory pressure will swap rather
                 * than free the pages we are allocating (and SGP_CACHE pages
                 * might still be clean: we now need to mark those dirty too).
                 */
                set_page_dirty(page);
                unlock_page(page);
                page_cache_release(page);
                cond_resched();
        }

        if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
                i_size_write(inode, offset + len);
        inode->i_ctime = CURRENT_TIME;
undone:
        spin_lock(&inode->i_lock);
        inode->i_private = NULL;
        spin_unlock(&inode->i_lock);
out:
        mutex_unlock(&inode->i_mutex);
        return error;
}

static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);

        buf->f_type = TMPFS_MAGIC;
        buf->f_bsize = PAGE_CACHE_SIZE;
        buf->f_namelen = NAME_MAX;
        if (sbinfo->max_blocks) {
                buf->f_blocks = sbinfo->max_blocks;
                buf->f_bavail =
                buf->f_bfree  = sbinfo->max_blocks -
                                percpu_counter_sum(&sbinfo->used_blocks);
        }
        if (sbinfo->max_inodes) {
                buf->f_files = sbinfo->max_inodes;
                buf->f_ffree = sbinfo->free_inodes;
        }
        /* else leave those fields 0 like simple_statfs */
        return 0;
}

/*
 * File creation. Allocate an inode, and we're done..
 */
static int
shmem_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
        struct inode *inode;
        int error = -ENOSPC;

        inode = shmem_get_inode(dir->i_sb, dir, mode, dev, VM_NORESERVE);
        if (inode) {
                error = simple_acl_create(dir, inode);
                if (error)
                        goto out_iput;
                error = security_inode_init_security(inode, dir,
                                                     &dentry->d_name,
                                                     shmem_initxattrs, NULL);
                if (error && error != -EOPNOTSUPP)
                        goto out_iput;

                error = 0;
                dir->i_size += BOGO_DIRENT_SIZE;
                dir->i_ctime = dir->i_mtime = CURRENT_TIME;
                d_instantiate(dentry, inode);
                dget(dentry); /* Extra count - pin the dentry in core */
        }
        return error;
out_iput:
        iput(inode);
        return error;
}

static int
shmem_tmpfile(struct inode *dir, struct dentry *dentry, umode_t mode)
{
        struct inode *inode;
        int error = -ENOSPC;

        inode = shmem_get_inode(dir->i_sb, dir, mode, 0, VM_NORESERVE);
        if (inode) {
                error = security_inode_init_security(inode, dir,
                                                     NULL,
                                                     shmem_initxattrs, NULL);
                if (error && error != -EOPNOTSUPP)
                        goto out_iput;
                error = simple_acl_create(dir, inode);
                if (error)
                        goto out_iput;
                d_tmpfile(dentry, inode);
        }
        return error;
out_iput:
        iput(inode);
        return error;
}

static int shmem_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
{
        int error;

        if ((error = shmem_mknod(dir, dentry, mode | S_IFDIR, 0)))
                return error;
        inc_nlink(dir);
        return 0;
}

static int shmem_create(struct inode *dir, struct dentry *dentry, umode_t mode,
                bool excl)
{
        return shmem_mknod(dir, dentry, mode | S_IFREG, 0);
}

/*
 * Link a file..
 */
static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = old_dentry->d_inode;
        int ret;

        /*
         * No ordinary (disk based) filesystem counts links as inodes;
         * but each new link needs a new dentry, pinning lowmem, and
         * tmpfs dentries cannot be pruned until they are unlinked.
         */
        ret = shmem_reserve_inode(inode->i_sb);
        if (ret)
                goto out;

        dir->i_size += BOGO_DIRENT_SIZE;
        inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        inc_nlink(inode);
        ihold(inode);   /* New dentry reference */
        dget(dentry);           /* Extra pinning count for the created dentry */
        d_instantiate(dentry, inode);
out:
        return ret;
}

static int shmem_unlink(struct inode *dir, struct dentry *dentry)
{
        struct inode *inode = dentry->d_inode;

        if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
                shmem_free_inode(inode->i_sb);

        dir->i_size -= BOGO_DIRENT_SIZE;
        inode->i_ctime = dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        drop_nlink(inode);
        dput(dentry);   /* Undo the count from "create" - this does all the work */
        return 0;
}

static int shmem_rmdir(struct inode *dir, struct dentry *dentry)
{
        if (!simple_empty(dentry))
                return -ENOTEMPTY;

        drop_nlink(dentry->d_inode);
        drop_nlink(dir);
        return shmem_unlink(dir, dentry);
}

/*
 * The VFS layer already does all the dentry stuff for rename,
 * we just have to decrement the usage count for the target if
 * it exists so that the VFS layer correctly free's it when it
 * gets overwritten.
 */
static int shmem_rename(struct inode *old_dir, struct dentry *old_dentry, struct inode *new_dir, struct dentry *new_dentry)
{
        struct inode *inode = old_dentry->d_inode;
        int they_are_dirs = S_ISDIR(inode->i_mode);

        if (!simple_empty(new_dentry))
                return -ENOTEMPTY;

        if (new_dentry->d_inode) {
                (void) shmem_unlink(new_dir, new_dentry);
                if (they_are_dirs)
                        drop_nlink(old_dir);
        } else if (they_are_dirs) {
                drop_nlink(old_dir);
                inc_nlink(new_dir);
        }

        old_dir->i_size -= BOGO_DIRENT_SIZE;
        new_dir->i_size += BOGO_DIRENT_SIZE;
        old_dir->i_ctime = old_dir->i_mtime =
        new_dir->i_ctime = new_dir->i_mtime =
        inode->i_ctime = CURRENT_TIME;
        return 0;
}

static int shmem_symlink(struct inode *dir, struct dentry *dentry, const char *symname)
{
        int error;
        int len;
        struct inode *inode;
        struct page *page;
        char *kaddr;
        struct shmem_inode_info *info;

        len = strlen(symname) + 1;
        if (len > PAGE_CACHE_SIZE)
                return -ENAMETOOLONG;

        inode = shmem_get_inode(dir->i_sb, dir, S_IFLNK|S_IRWXUGO, 0, VM_NORESERVE);
        if (!inode)
                return -ENOSPC;

        error = security_inode_init_security(inode, dir, &dentry->d_name,
                                             shmem_initxattrs, NULL);
        if (error) {
                if (error != -EOPNOTSUPP) {
                        iput(inode);
                        return error;
                }
                error = 0;
        }

        info = SHMEM_I(inode);
        inode->i_size = len-1;
        if (len <= SHORT_SYMLINK_LEN) {
                info->symlink = kmemdup(symname, len, GFP_KERNEL);
                if (!info->symlink) {
                        iput(inode);
                        return -ENOMEM;
                }
                inode->i_op = &shmem_short_symlink_operations;
        } else {
                error = shmem_getpage(inode, 0, &page, SGP_WRITE, NULL);
                if (error) {
                        iput(inode);
                        return error;
                }
                inode->i_mapping->a_ops = &shmem_aops;
                inode->i_op = &shmem_symlink_inode_operations;
                kaddr = kmap_atomic(page);
                memcpy(kaddr, symname, len);
                kunmap_atomic(kaddr);
                SetPageUptodate(page);
                set_page_dirty(page);
                unlock_page(page);
                page_cache_release(page);
        }
        dir->i_size += BOGO_DIRENT_SIZE;
        dir->i_ctime = dir->i_mtime = CURRENT_TIME;
        d_instantiate(dentry, inode);
        dget(dentry);
        return 0;
}

static void *shmem_follow_short_symlink(struct dentry *dentry, struct nameidata *nd)
{
        nd_set_link(nd, SHMEM_I(dentry->d_inode)->symlink);
        return NULL;
}

static void *shmem_follow_link(struct dentry *dentry, struct nameidata *nd)
{
        struct page *page = NULL;
        int error = shmem_getpage(dentry->d_inode, 0, &page, SGP_READ, NULL);
        nd_set_link(nd, error ? ERR_PTR(error) : kmap(page));
        if (page)
                unlock_page(page);
        return page;
}

static void shmem_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
{
        if (!IS_ERR(nd_get_link(nd))) {
                struct page *page = cookie;
                kunmap(page);
                mark_page_accessed(page);
                page_cache_release(page);
        }
}

#ifdef CONFIG_TMPFS_XATTR
/*
 * Superblocks without xattr inode operations may get some security.* xattr
 * support from the LSM "for free". As soon as we have any other xattrs
 * like ACLs, we also need to implement the security.* handlers at
 * filesystem level, though.
 */

/*
 * Callback for security_inode_init_security() for acquiring xattrs.
 */
static int shmem_initxattrs(struct inode *inode,
                            const struct xattr *xattr_array,
                            void *fs_info)
{
        struct shmem_inode_info *info = SHMEM_I(inode);
        const struct xattr *xattr;
        struct simple_xattr *new_xattr;
        size_t len;

        for (xattr = xattr_array; xattr->name != NULL; xattr++) {
                new_xattr = simple_xattr_alloc(xattr->value, xattr->value_len);
                if (!new_xattr)
                        return -ENOMEM;

                len = strlen(xattr->name) + 1;
                new_xattr->name = kmalloc(XATTR_SECURITY_PREFIX_LEN + len,
                                          GFP_KERNEL);
                if (!new_xattr->name) {
                        kfree(new_xattr);
                        return -ENOMEM;
                }

                memcpy(new_xattr->name, XATTR_SECURITY_PREFIX,
                       XATTR_SECURITY_PREFIX_LEN);
                memcpy(new_xattr->name + XATTR_SECURITY_PREFIX_LEN,
                       xattr->name, len);

                simple_xattr_list_add(&info->xattrs, new_xattr);
        }

        return 0;
}

static const struct xattr_handler *shmem_xattr_handlers[] = {
#ifdef CONFIG_TMPFS_POSIX_ACL
        &posix_acl_access_xattr_handler,
        &posix_acl_default_xattr_handler,
#endif
        NULL
};

static int shmem_xattr_validate(const char *name)
{
        struct { const char *prefix; size_t len; } arr[] = {
                { XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN },
                { XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN }
        };
        int i;

        for (i = 0; i < ARRAY_SIZE(arr); i++) {
                size_t preflen = arr[i].len;
                if (strncmp(name, arr[i].prefix, preflen) == 0) {
                        if (!name[preflen])
                                return -EINVAL;
                        return 0;
                }
        }
        return -EOPNOTSUPP;
}

static ssize_t shmem_getxattr(struct dentry *dentry, const char *name,
                              void *buffer, size_t size)
{
        struct shmem_inode_info *info = SHMEM_I(dentry->d_inode);
        int err;

        /*
         * If this is a request for a synthetic attribute in the system.*
         * namespace use the generic infrastructure to resolve a handler
         * for it via sb->s_xattr.
         */
        if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
                return generic_getxattr(dentry, name, buffer, size);

        err = shmem_xattr_validate(name);
        if (err)
                return err;

        return simple_xattr_get(&info->xattrs, name, buffer, size);
}

static int shmem_setxattr(struct dentry *dentry, const char *name,
                          const void *value, size_t size, int flags)
{
        struct shmem_inode_info *info = SHMEM_I(dentry->d_inode);
        int err;

        /*
         * If this is a request for a synthetic attribute in the system.*
         * namespace use the generic infrastructure to resolve a handler
         * for it via sb->s_xattr.
         */
        if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
                return generic_setxattr(dentry, name, value, size, flags);

        err = shmem_xattr_validate(name);
        if (err)
                return err;

        return simple_xattr_set(&info->xattrs, name, value, size, flags);
}

static int shmem_removexattr(struct dentry *dentry, const char *name)
{
        struct shmem_inode_info *info = SHMEM_I(dentry->d_inode);
        int err;

        /*
         * If this is a request for a synthetic attribute in the system.*
         * namespace use the generic infrastructure to resolve a handler
         * for it via sb->s_xattr.
         */
        if (!strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
                return generic_removexattr(dentry, name);

        err = shmem_xattr_validate(name);
        if (err)
                return err;

        return simple_xattr_remove(&info->xattrs, name);
}

static ssize_t shmem_listxattr(struct dentry *dentry, char *buffer, size_t size)
{
        struct shmem_inode_info *info = SHMEM_I(dentry->d_inode);
        return simple_xattr_list(&info->xattrs, buffer, size);
}
#endif /* CONFIG_TMPFS_XATTR */

static const struct inode_operations shmem_short_symlink_operations = {
        .readlink       = generic_readlink,
        .follow_link    = shmem_follow_short_symlink,
#ifdef CONFIG_TMPFS_XATTR
        .setxattr       = shmem_setxattr,
        .getxattr       = shmem_getxattr,
        .listxattr      = shmem_listxattr,
        .removexattr    = shmem_removexattr,
#endif
};

static const struct inode_operations shmem_symlink_inode_operations = {
        .readlink       = generic_readlink,
        .follow_link    = shmem_follow_link,
        .put_link       = shmem_put_link,
#ifdef CONFIG_TMPFS_XATTR
        .setxattr       = shmem_setxattr,
        .getxattr       = shmem_getxattr,
        .listxattr      = shmem_listxattr,
        .removexattr    = shmem_removexattr,
#endif
};

static struct dentry *shmem_get_parent(struct dentry *child)
{
        return ERR_PTR(-ESTALE);
}

static int shmem_match(struct inode *ino, void *vfh)
{
        __u32 *fh = vfh;
        __u64 inum = fh[2];
        inum = (inum << 32) | fh[1];
        return ino->i_ino == inum && fh[0] == ino->i_generation;
}

static struct dentry *shmem_fh_to_dentry(struct super_block *sb,
                struct fid *fid, int fh_len, int fh_type)
{
        struct inode *inode;
        struct dentry *dentry = NULL;
        u64 inum;

        if (fh_len < 3)
                return NULL;

        inum = fid->raw[2];
        inum = (inum << 32) | fid->raw[1];

        inode = ilookup5(sb, (unsigned long)(inum + fid->raw[0]),
                        shmem_match, fid->raw);
        if (inode) {
                dentry = d_find_alias(inode);
                iput(inode);
        }

        return dentry;
}

static int shmem_encode_fh(struct inode *inode, __u32 *fh, int *len,
                                struct inode *parent)
{
        if (*len < 3) {
                *len = 3;
                return FILEID_INVALID;
        }

        if (inode_unhashed(inode)) {
                /* Unfortunately insert_inode_hash is not idempotent,
                 * so as we hash inodes here rather than at creation
                 * time, we need a lock to ensure we only try
                 * to do it once
                 */
                static DEFINE_SPINLOCK(lock);
                spin_lock(&lock);
                if (inode_unhashed(inode))
                        __insert_inode_hash(inode,
                                            inode->i_ino + inode->i_generation);
                spin_unlock(&lock);
        }

        fh[0] = inode->i_generation;
        fh[1] = inode->i_ino;
        fh[2] = ((__u64)inode->i_ino) >> 32;

        *len = 3;
        return 1;
}

static const struct export_operations shmem_export_ops = {
        .get_parent     = shmem_get_parent,
        .encode_fh      = shmem_encode_fh,
        .fh_to_dentry   = shmem_fh_to_dentry,
};

static int shmem_parse_options(char *options, struct shmem_sb_info *sbinfo,
                               bool remount)
{
        char *this_char, *value, *rest;
        struct mempolicy *mpol = NULL;
        uid_t uid;
        gid_t gid;

        while (options != NULL) {
                this_char = options;
                for (;;) {
                        /*
                         * NUL-terminate this option: unfortunately,
                         * mount options form a comma-separated list,
                         * but mpol's nodelist may also contain commas.
                         */
                        options = strchr(options, ',');
                        if (options == NULL)
                                break;
                        options++;
                        if (!isdigit(*options)) {
                                options[-1] = '\0';
                                break;
                        }
                }
                if (!*this_char)
                        continue;
                if ((value = strchr(this_char,'=')) != NULL) {
                        *value++ = 0;
                } else {
                        printk(KERN_ERR
                            "tmpfs: No value for mount option '%s'\n",
                            this_char);
                        goto error;
                }

                if (!strcmp(this_char,"size")) {
                        unsigned long long size;
                        size = memparse(value,&rest);
                        if (*rest == '%') {
                                size <<= PAGE_SHIFT;
                                size *= totalram_pages;
                                do_div(size, 100);
                                rest++;
                        }
                        if (*rest)
                                goto bad_val;
                        sbinfo->max_blocks =
                                DIV_ROUND_UP(size, PAGE_CACHE_SIZE);
                } else if (!strcmp(this_char,"nr_blocks")) {
                        sbinfo->max_blocks = memparse(value, &rest);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"nr_inodes")) {
                        sbinfo->max_inodes = memparse(value, &rest);
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"mode")) {
                        if (remount)
                                continue;
                        sbinfo->mode = simple_strtoul(value, &rest, 8) & 07777;
                        if (*rest)
                                goto bad_val;
                } else if (!strcmp(this_char,"uid")) {
                        if (remount)
                                continue;
                        uid = simple_strtoul(value, &rest, 0);
                        if (*rest)
                                goto bad_val;
                        sbinfo->uid = make_kuid(current_user_ns(), uid);
                        if (!uid_valid(sbinfo->uid))
                                goto bad_val;
                } else if (!strcmp(this_char,"gid")) {
                        if (remount)
                                continue;
                        gid = simple_strtoul(value, &rest, 0);
                        if (*rest)
                                goto bad_val;
                        sbinfo->gid = make_kgid(current_user_ns(), gid);
                        if (!gid_valid(sbinfo->gid))
                                goto bad_val;
                } else if (!strcmp(this_char,"mpol")) {
                        mpol_put(mpol);
                        mpol = NULL;
                        if (mpol_parse_str(value, &mpol))
                                goto bad_val;
                } else {
                        printk(KERN_ERR "tmpfs: Bad mount option %s\n",
                               this_char);
                        goto error;
                }
        }
        sbinfo->mpol = mpol;
        return 0;

bad_val:
        printk(KERN_ERR "tmpfs: Bad value '%s' for mount option '%s'\n",
               value, this_char);
error:
        mpol_put(mpol);
        return 1;

}

static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
        struct shmem_sb_info config = *sbinfo;
        unsigned long inodes;
        int error = -EINVAL;

        config.mpol = NULL;
        if (shmem_parse_options(data, &config, true))
                return error;

        spin_lock(&sbinfo->stat_lock);
        inodes = sbinfo->max_inodes - sbinfo->free_inodes;
        if (percpu_counter_compare(&sbinfo->used_blocks, config.max_blocks) > 0)
                goto out;
        if (config.max_inodes < inodes)
                goto out;
        /*
         * Those tests disallow limited->unlimited while any are in use;
         * but we must separately disallow unlimited->limited, because
         * in that case we have no record of how much is already in use.
         */
        if (config.max_blocks && !sbinfo->max_blocks)
                goto out;
        if (config.max_inodes && !sbinfo->max_inodes)
                goto out;

        error = 0;
        sbinfo->max_blocks  = config.max_blocks;
        sbinfo->max_inodes  = config.max_inodes;
        sbinfo->free_inodes = config.max_inodes - inodes;

        /*
         * Preserve previous mempolicy unless mpol remount option was specified.
         */
        if (config.mpol) {
                mpol_put(sbinfo->mpol);
                sbinfo->mpol = config.mpol;     /* transfers initial ref */
        }
out:
        spin_unlock(&sbinfo->stat_lock);
        return error;
}

static int shmem_show_options(struct seq_file *seq, struct dentry *root)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(root->d_sb);

        if (sbinfo->max_blocks != shmem_default_max_blocks())
                seq_printf(seq, ",size=%luk",
                        sbinfo->max_blocks << (PAGE_CACHE_SHIFT - 10));
        if (sbinfo->max_inodes != shmem_default_max_inodes())
                seq_printf(seq, ",nr_inodes=%lu", sbinfo->max_inodes);
        if (sbinfo->mode != (S_IRWXUGO | S_ISVTX))
                seq_printf(seq, ",mode=%03ho", sbinfo->mode);
        if (!uid_eq(sbinfo->uid, GLOBAL_ROOT_UID))
                seq_printf(seq, ",uid=%u",
                                from_kuid_munged(&init_user_ns, sbinfo->uid));
        if (!gid_eq(sbinfo->gid, GLOBAL_ROOT_GID))
                seq_printf(seq, ",gid=%u",
                                from_kgid_munged(&init_user_ns, sbinfo->gid));
        shmem_show_mpol(seq, sbinfo->mpol);
        return 0;
}
#endif /* CONFIG_TMPFS */

static void shmem_put_super(struct super_block *sb)
{
        struct shmem_sb_info *sbinfo = SHMEM_SB(sb);

        percpu_counter_destroy(&sbinfo->used_blocks);
        mpol_put(sbinfo->mpol);
        kfree(sbinfo);
        sb->s_fs_info = NULL;
}

int shmem_fill_super(struct super_block *sb, void *data, int silent)
{
        struct inode *inode;
        struct shmem_sb_info *sbinfo;
        int err = -ENOMEM;

        /* Round up to L1_CACHE_BYTES to resist false sharing */
        sbinfo = kzalloc(max((int)sizeof(struct shmem_sb_info),
                                L1_CACHE_BYTES), GFP_KERNEL);
        if (!sbinfo)
                return -ENOMEM;

        sbinfo->mode = S_IRWXUGO | S_ISVTX;
        sbinfo->uid = current_fsuid();
        sbinfo->gid = current_fsgid();
        sb->s_fs_info = sbinfo;

#ifdef CONFIG_TMPFS
        /*
         * Per default we only allow half of the physical ram per
         * tmpfs instance, limiting inodes to one per page of lowmem;
         * but the internal instance is left unlimited.
         */
        if (!(sb->s_flags & MS_KERNMOUNT)) {
                sbinfo->max_blocks = shmem_default_max_blocks();
                sbinfo->max_inodes = shmem_default_max_inodes();
                if (shmem_parse_options(data, sbinfo, false)) {
                        err = -EINVAL;
                        goto failed;
                }
        } else {
                sb->s_flags |= MS_NOUSER;
        }
        sb->s_export_op = &shmem_export_ops;
        sb->s_flags |= MS_NOSEC;
#else
        sb->s_flags |= MS_NOUSER;
#endif

        spin_lock_init(&sbinfo->stat_lock);
        if (percpu_counter_init(&sbinfo->used_blocks, 0))
                goto failed;
        sbinfo->free_inodes = sbinfo->max_inodes;

        sb->s_maxbytes = MAX_LFS_FILESIZE;
        sb->s_blocksize = PAGE_CACHE_SIZE;
        sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
        sb->s_magic = TMPFS_MAGIC;
        sb->s_op = &shmem_ops;
        sb->s_time_gran = 1;
#ifdef CONFIG_TMPFS_XATTR
        sb->s_xattr = shmem_xattr_handlers;
#endif
#ifdef CONFIG_TMPFS_POSIX_ACL
        sb->s_flags |= MS_POSIXACL;
#endif

        inode = shmem_get_inode(sb, NULL, S_IFDIR | sbinfo->mode, 0, VM_NORESERVE);
        if (!inode)
                goto failed;
        inode->i_uid = sbinfo->uid;
        inode->i_gid = sbinfo->gid;
        sb->s_root = d_make_root(inode);
        if (!sb->s_root)
                goto failed;
        return 0;

failed:
        shmem_put_super(sb);
        return err;
}

static struct kmem_cache *shmem_inode_cachep;

static struct inode *shmem_alloc_inode(struct super_block *sb)
{
        struct shmem_inode_info *info;
        info = kmem_cache_alloc(shmem_inode_cachep, GFP_KERNEL);
        if (!info)
                return NULL;
        return &info->vfs_inode;
}

static void shmem_destroy_callback(struct rcu_head *head)
{
        struct inode *inode = container_of(head, struct inode, i_rcu);
        kmem_cache_free(shmem_inode_cachep, SHMEM_I(inode));
}

static void shmem_destroy_inode(struct inode *inode)
{
        if (S_ISREG(inode->i_mode))
                mpol_free_shared_policy(&SHMEM_I(inode)->policy);
        call_rcu(&inode->i_rcu, shmem_destroy_callback);
}

static void shmem_init_inode(void *foo)
{
        struct shmem_inode_info *info = foo;
        inode_init_once(&info->vfs_inode);
}

static int shmem_init_inodecache(void)
{
        shmem_inode_cachep = kmem_cache_create("shmem_inode_cache",
                                sizeof(struct shmem_inode_info),
                                0, SLAB_PANIC, shmem_init_inode);
        return 0;
}

static void shmem_destroy_inodecache(void)
{
        kmem_cache_destroy(shmem_inode_cachep);
}

static const struct address_space_operations shmem_aops = {
        .writepage      = shmem_writepage,
        .set_page_dirty = __set_page_dirty_no_writeback,
#ifdef CONFIG_TMPFS
        .write_begin    = shmem_write_begin,
        .write_end      = shmem_write_end,
#endif
        .migratepage    = migrate_page,
        .error_remove_page = generic_error_remove_page,
};

static const struct file_operations shmem_file_operations = {
        .mmap           = shmem_mmap,
#ifdef CONFIG_TMPFS
        .llseek         = shmem_file_llseek,
        .read           = new_sync_read,
        .write          = new_sync_write,
        .read_iter      = shmem_file_read_iter,
        .write_iter     = generic_file_write_iter,
        .fsync          = noop_fsync,
        .splice_read    = shmem_file_splice_read,
        .splice_write   = iter_file_splice_write,
        .fallocate      = shmem_fallocate,
#endif
};

static const struct inode_operations shmem_inode_operations = {
        .setattr        = shmem_setattr,
#ifdef CONFIG_TMPFS_XATTR
        .setxattr       = shmem_setxattr,
        .getxattr       = shmem_getxattr,
        .listxattr      = shmem_listxattr,
        .removexattr    = shmem_removexattr,
        .set_acl        = simple_set_acl,
#endif
};

static const struct inode_operations shmem_dir_inode_operations = {
#ifdef CONFIG_TMPFS
        .create         = shmem_create,
        .lookup         = simple_lookup,
        .link           = shmem_link,
        .unlink         = shmem_unlink,
        .symlink        = shmem_symlink,
        .mkdir          = shmem_mkdir,
        .rmdir          = shmem_rmdir,
        .mknod          = shmem_mknod,
        .rename         = shmem_rename,
        .tmpfile        = shmem_tmpfile,
#endif
#ifdef CONFIG_TMPFS_XATTR
        .setxattr       = shmem_setxattr,
        .getxattr       = shmem_getxattr,
        .listxattr      = shmem_listxattr,
        .removexattr    = shmem_removexattr,
#endif
#ifdef CONFIG_TMPFS_POSIX_ACL
        .setattr        = shmem_setattr,
        .set_acl        = simple_set_acl,
#endif
};

static const struct inode_operations shmem_special_inode_operations = {
#ifdef CONFIG_TMPFS_XATTR
        .setxattr       = shmem_setxattr,
        .getxattr       = shmem_getxattr,
        .listxattr      = shmem_listxattr,
        .removexattr    = shmem_removexattr,
#endif
#ifdef CONFIG_TMPFS_POSIX_ACL
        .setattr        = shmem_setattr,
        .set_acl        = simple_set_acl,
#endif
};

static const struct super_operations shmem_ops = {
        .alloc_inode    = shmem_alloc_inode,
        .destroy_inode  = shmem_destroy_inode,
#ifdef CONFIG_TMPFS
        .statfs         = shmem_statfs,
        .remount_fs     = shmem_remount_fs,
        .show_options   = shmem_show_options,
#endif
        .evict_inode    = shmem_evict_inode,
        .drop_inode     = generic_delete_inode,
        .put_super      = shmem_put_super,
};

static const struct vm_operations_struct shmem_vm_ops = {
        .fault          = shmem_fault,
        .map_pages      = filemap_map_pages,
#ifdef CONFIG_NUMA
        .set_policy     = shmem_set_policy,
        .get_policy     = shmem_get_policy,
#endif
        .remap_pages    = generic_file_remap_pages,
};

static struct dentry *shmem_mount(struct file_system_type *fs_type,
        int flags, const char *dev_name, void *data)
{
        return mount_nodev(fs_type, flags, data, shmem_fill_super);
}

static struct file_system_type shmem_fs_type = {
        .owner          = THIS_MODULE,
        .name           = "tmpfs",
        .mount          = shmem_mount,
        .kill_sb        = kill_litter_super,
        .fs_flags       = FS_USERNS_MOUNT,
};

int __init shmem_init(void)
{
        int error;

        /* If rootfs called this, don't re-init */
        if (shmem_inode_cachep)
                return 0;

        error = bdi_init(&shmem_backing_dev_info);
        if (error)
                goto out4;

        error = shmem_init_inodecache();
        if (error)
                goto out3;

        error = register_filesystem(&shmem_fs_type);
        if (error) {
                printk(KERN_ERR "Could not register tmpfs\n");
                goto out2;
        }

        shm_mnt = kern_mount(&shmem_fs_type);
        if (IS_ERR(shm_mnt)) {
                error = PTR_ERR(shm_mnt);
                printk(KERN_ERR "Could not kern_mount tmpfs\n");
                goto out1;
        }
        return 0;

out1:
        unregister_filesystem(&shmem_fs_type);
out2:
        shmem_destroy_inodecache();
out3:
        bdi_destroy(&shmem_backing_dev_info);
out4:
        shm_mnt = ERR_PTR(error);
        return error;
}

#else /* !CONFIG_SHMEM */

/*
 * tiny-shmem: simple shmemfs and tmpfs using ramfs code
 *
 * This is intended for small system where the benefits of the full
 * shmem code (swap-backed and resource-limited) are outweighed by
 * their complexity. On systems without swap this code should be
 * effectively equivalent, but much lighter weight.
 */

static struct file_system_type shmem_fs_type = {
        .name           = "tmpfs",
        .mount          = ramfs_mount,
        .kill_sb        = kill_litter_super,
        .fs_flags       = FS_USERNS_MOUNT,
};

int __init shmem_init(void)
{
        BUG_ON(register_filesystem(&shmem_fs_type) != 0);

        shm_mnt = kern_mount(&shmem_fs_type);
        BUG_ON(IS_ERR(shm_mnt));

        return 0;
}

int shmem_unuse(swp_entry_t swap, struct page *page)
{
        return 0;
}

int shmem_lock(struct file *file, int lock, struct user_struct *user)
{
        return 0;
}

void shmem_unlock_mapping(struct address_space *mapping)
{
}

void shmem_truncate_range(struct inode *inode, loff_t lstart, loff_t lend)
{
        truncate_inode_pages_range(inode->i_mapping, lstart, lend);
}
EXPORT_SYMBOL_GPL(shmem_truncate_range);

#define shmem_vm_ops                            generic_file_vm_ops
#define shmem_file_operations                   ramfs_file_operations
#define shmem_get_inode(sb, dir, mode, dev, flags)      ramfs_get_inode(sb, dir, mode, dev)
#define shmem_acct_size(flags, size)            0
#define shmem_unacct_size(flags, size)          do {} while (0)

#endif /* CONFIG_SHMEM */

/* common code */

static struct dentry_operations anon_ops = {
        .d_dname = simple_dname
};

static struct file *__shmem_file_setup(const char *name, loff_t size,
                                       unsigned long flags, unsigned int i_flags)
{
        struct file *res;
        struct inode *inode;
        struct path path;
        struct super_block *sb;
        struct qstr this;

        if (IS_ERR(shm_mnt))
                return ERR_CAST(shm_mnt);

        if (size < 0 || size > MAX_LFS_FILESIZE)
                return ERR_PTR(-EINVAL);

        if (shmem_acct_size(flags, size))
                return ERR_PTR(-ENOMEM);

        res = ERR_PTR(-ENOMEM);
        this.name = name;
        this.len = strlen(name);
        this.hash = 0; /* will go */
        sb = shm_mnt->mnt_sb;
        path.dentry = d_alloc_pseudo(sb, &this);
        if (!path.dentry)
                goto put_memory;
        d_set_d_op(path.dentry, &anon_ops);
        path.mnt = mntget(shm_mnt);

        res = ERR_PTR(-ENOSPC);
        inode = shmem_get_inode(sb, NULL, S_IFREG | S_IRWXUGO, 0, flags);
        if (!inode)
                goto put_dentry;

        inode->i_flags |= i_flags;
        d_instantiate(path.dentry, inode);
        inode->i_size = size;
        clear_nlink(inode);     /* It is unlinked */
        res = ERR_PTR(ramfs_nommu_expand_for_mapping(inode, size));
        if (IS_ERR(res))
                goto put_dentry;

        res = alloc_file(&path, FMODE_WRITE | FMODE_READ,
                  &shmem_file_operations);
        if (IS_ERR(res))
                goto put_dentry;

        return res;

put_dentry:
        path_put(&path);
put_memory:
        shmem_unacct_size(flags, size);
        return res;
}

/**
 * shmem_kernel_file_setup - get an unlinked file living in tmpfs which must be
 *      kernel internal.  There will be NO LSM permission checks against the
 *      underlying inode.  So users of this interface must do LSM checks at a
 *      higher layer.  The one user is the big_key implementation.  LSM checks
 *      are provided at the key level rather than the inode level.
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
 */
struct file *shmem_kernel_file_setup(const char *name, loff_t size, unsigned long flags)
{
        return __shmem_file_setup(name, size, flags, S_PRIVATE);
}

/**
 * shmem_file_setup - get an unlinked file living in tmpfs
 * @name: name for dentry (to be seen in /proc/<pid>/maps
 * @size: size to be set for the file
 * @flags: VM_NORESERVE suppresses pre-accounting of the entire object size
 */
struct file *shmem_file_setup(const char *name, loff_t size, unsigned long flags)
{
        return __shmem_file_setup(name, size, flags, 0);
}
EXPORT_SYMBOL_GPL(shmem_file_setup);

/**
 * shmem_zero_setup - setup a shared anonymous mapping
 * @vma: the vma to be mmapped is prepared by do_mmap_pgoff
 */
int shmem_zero_setup(struct vm_area_struct *vma)
{
        struct file *file;
        loff_t size = vma->vm_end - vma->vm_start;

        file = shmem_file_setup("dev/zero", size, vma->vm_flags);
        if (IS_ERR(file))
                return PTR_ERR(file);

        if (vma->vm_file)
                fput(vma->vm_file);
        vma->vm_file = file;
        vma->vm_ops = &shmem_vm_ops;
        return 0;
}

/**
 * shmem_read_mapping_page_gfp - read into page cache, using specified page allocation flags.
 * @mapping:    the page's address_space
 * @index:      the page index
 * @gfp:        the page allocator flags to use if allocating
 *
 * This behaves as a tmpfs "read_cache_page_gfp(mapping, index, gfp)",
 * with any new page allocations done using the specified allocation flags.
 * But read_cache_page_gfp() uses the ->readpage() method: which does not
 * suit tmpfs, since it may have pages in swapcache, and needs to find those
 * for itself; although drivers/gpu/drm i915 and ttm rely upon this support.
 *
 * i915_gem_object_get_pages_gtt() mixes __GFP_NORETRY | __GFP_NOWARN in
 * with the mapping_gfp_mask(), to avoid OOMing the machine unnecessarily.
 */
struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
                                         pgoff_t index, gfp_t gfp)
{
#ifdef CONFIG_SHMEM
        struct inode *inode = mapping->host;
        struct page *page;
        int error;

        BUG_ON(mapping->a_ops != &shmem_aops);
        error = shmem_getpage_gfp(inode, index, &page, SGP_CACHE, gfp, NULL);
        if (error)
                page = ERR_PTR(error);
        else
                unlock_page(page);
        return page;
#else
        /*
         * The tiny !SHMEM case uses ramfs without swap
         */
        return read_cache_page_gfp(mapping, index, gfp);
#endif
}
EXPORT_SYMBOL_GPL(shmem_read_mapping_page_gfp);