Convert shmem_unuse_inode() to use a lockless gang lookup of the radix

tree, searching for matching swap.

This is somewhat slower than the old method: because of repeated radix
tree descents, because of copying entries up, but probably most because
the old method noted and skipped once a vector page was cleared of swap.
Perhaps we can devise a use of radix tree tagging to achieve that later.

shmem_add_to_page_cache() uses shmem_radix_tree_replace() to compensate
for the lockless lookup by checking that the expected entry is in place,
under lock.  It is not very satisfactory to be copying this much from
add_to_page_cache_locked(), but I think easier to sell than insisting that
every caller of add_to_page_cache*() go through the extras.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 4439b7d55819c7ae849f561b09abc6c0ef0ea60f..174f97188e8a665dbfab40b506b3646f6fc8acdb 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -261,6 +261,55 @@ static int shmem_radix_tree_replace(struct address_space *mapping,
        return 0;
 }
 
+/*
+ * 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(!PageLocked(page));
+       VM_BUG_ON(!PageSwapBacked(page));
+
+       error = mem_cgroup_cache_charge(page, current->mm,
+                                               gfp & GFP_RECLAIM_MASK);
+       if (error)
+               goto out;
+       if (!expected)
+               error = radix_tree_preload(gfp & GFP_RECLAIM_MASK);
+       if (!error) {
+               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);
+               }
+               if (!expected)
+                       radix_tree_preload_end();
+       }
+       if (error)
+               mem_cgroup_uncharge_cache_page(page);
+out:
+       return error;
+}
+
 /*
  * Like find_get_pages, but collecting swap entries as well as pages.
  */
@@ -308,6 +357,42 @@ export:
        return ret;
 }
 
+/*
+ * Lockless lookup of swap entry in radix tree, avoiding refcount on pages.
+ */
+static pgoff_t shmem_find_swap(struct address_space *mapping, void *radswap)
+{
+       void  **slots[PAGEVEC_SIZE];
+       pgoff_t indices[PAGEVEC_SIZE];
+       unsigned int nr_found;
+
+restart:
+       nr_found = 1;
+       indices[0] = -1;
+       while (nr_found) {
+               pgoff_t index = indices[nr_found - 1] + 1;
+               unsigned int i;
+
+               rcu_read_lock();
+               nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
+                                       slots, indices, index, PAGEVEC_SIZE);
+               for (i = 0; i < nr_found; i++) {
+                       void *item = radix_tree_deref_slot(slots[i]);
+                       if (radix_tree_deref_retry(item)) {
+                               rcu_read_unlock();
+                               goto restart;
+                       }
+                       if (item == radswap) {
+                               rcu_read_unlock();
+                               return indices[i];
+                       }
+               }
+               rcu_read_unlock();
+               cond_resched();
+       }
+       return -1;
+}
+
 /*
  * Remove swap entry from radix tree, free the swap and its page cache.
  */
@@ -515,23 +600,21 @@ static void shmem_evict_inode(struct inode *inode)
        end_writeback(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 *page)
 {
        struct address_space *mapping = info->vfs_inode.i_mapping;
+       void *radswap;
        pgoff_t index;
        int error;
 
-       for (index = 0; index < SHMEM_NR_DIRECT; index++)
-               if (shmem_get_swap(info, index).val == swap.val)
-                       goto found;
-       return 0;
-found:
-       spin_lock(&info->lock);
-       if (shmem_get_swap(info, index).val != swap.val) {
-               spin_unlock(&info->lock);
+       radswap = swp_to_radix_entry(swap);
+       index = shmem_find_swap(mapping, radswap);
+       if (index == -1)
                return 0;
-       }
 
        /*
         * Move _head_ to start search for next from here.
@@ -547,23 +630,30 @@ found:
         * 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).
         */
-       error = add_to_page_cache_locked(page, mapping, index, GFP_NOWAIT);
+       error = shmem_add_to_page_cache(page, mapping, index,
+                                               GFP_NOWAIT, radswap);
        /* which does mem_cgroup_uncharge_cache_page on error */
 
        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(page);
                set_page_dirty(page);
-               shmem_put_swap(info, index, (swp_entry_t){0});
-               info->swapped--;
-               swap_free(swap);
+               if (!error) {
+                       spin_lock(&info->lock);
+                       info->swapped--;
+                       spin_unlock(&info->lock);
+                       swap_free(swap);
+               }
                error = 1;      /* not an error, but entry was found */
        }
-       spin_unlock(&info->lock);
        return error;
 }
 
 /*
- * shmem_unuse() search for an eventually swapped out shmem page.
+ * Search through swapped inodes to find and replace swap by page.
  */
 int shmem_unuse(swp_entry_t swap, struct page *page)
 {
@@ -576,20 +666,12 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
         * 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.
-        * add_to_page_cache() will be called with GFP_NOWAIT.
+        * shmem_add_to_page_cache() will be called with GFP_NOWAIT.
         */
        error = mem_cgroup_cache_charge(page, current->mm, GFP_KERNEL);
        if (error)
                goto out;
-       /*
-        * Try to preload while we can wait, to not make a habit of
-        * draining atomic reserves; but don't latch on to this cpu,
-        * it's okay if sometimes we get rescheduled after this.
-        */
-       error = radix_tree_preload(GFP_KERNEL);
-       if (error)
-               goto uncharge;
-       radix_tree_preload_end();
+       /* 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) {
@@ -608,7 +690,6 @@ int shmem_unuse(swp_entry_t swap, struct page *page)
        }
        mutex_unlock(&shmem_swaplist_mutex);
 
-uncharge:
        if (!found)
                mem_cgroup_uncharge_cache_page(page);
        if (found < 0)