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+
+Pathname lookup in Linux.
+=========================
+
+This write-up is based on three articles published at lwn.net:
+
+- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
+- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
+- <https://lwn.net/Articles/650786/> A walk among the symlinks
+
+Written by Neil Brown with help from Al Viro and Jon Corbet.
+
+Introduction
+------------
+
+The most obvious aspect of pathname lookup, which very little
+exploration is needed to discover, is that it is complex. There are
+many rules, special cases, and implementation alternatives that all
+combine to confuse the unwary reader. Computer science has long been
+acquainted with such complexity and has tools to help manage it. One
+tool that we will make extensive use of is "divide and conquer". For
+the early parts of the analysis we will divide off symlinks - leaving
+them until the final part. Well before we get to symlinks we have
+another major division based on the VFS's approach to locking which
+will allow us to review "REF-walk" and "RCU-walk" separately. But we
+are getting ahead of ourselves. There are some important low level
+distinctions we need to clarify first.
+
+There are two sorts of ...
+--------------------------
+
+[`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html
+
+Pathnames (sometimes "file names"), used to identify objects in the
+filesystem, will be familiar to most readers. They contain two sorts
+of elements: "slashes" that are sequences of one or more "`/`"
+characters, and "components" that are sequences of one or more
+non-"`/`" characters. These form two kinds of paths. Those that
+start with slashes are "absolute" and start from the filesystem root.
+The others are "relative" and start from the current directory, or
+from some other location specified by a file descriptor given to a
+"xxx`at`" system call such as "[`openat()`]".
+
+[`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html
+
+It is tempting to describe the second kind as starting with a
+component, but that isn't always accurate: a pathname can lack both
+slashes and components, it can be empty, in other words. This is
+generally forbidden in POSIX, but some of those "xxx`at`" system calls
+in Linux permit it when the `AT_EMPTY_PATH` flag is given. For
+example, if you have an open file descriptor on an executable file you
+can execute it by calling [`execveat()`] passing the file descriptor,
+an empty path, and the `AT_EMPTY_PATH` flag.
+
+These paths can be divided into two sections: the final component and
+everything else. The "everything else" is the easy bit. In all cases
+it must identify a directory that already exists, otherwise an error
+such as `ENOENT` or `ENOTDIR` will be reported.
+
+The final component is not so simple. Not only do different system
+calls interpret it quite differently (e.g. some create it, some do
+not), but it might not even exist: neither the empty pathname nor the
+pathname that is just slashes have a final component. If it does
+exist, it could be "`.`" or "`..`" which are handled quite differently
+from other components.
+
+[POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
+
+If a pathname ends with a slash, such as "`/tmp/foo/`" it might be
+tempting to consider that to have an empty final component. In many
+ways that would lead to correct results, but not always. In
+particular, `mkdir()` and `rmdir()` each create or remove a directory named
+by the final component, and they are required to work with pathnames
+ending in "`/`". According to [POSIX]
+
+> A pathname that contains at least one non- <slash> character and
+> that ends with one or more trailing <slash> characters shall not
+> be resolved successfully unless the last pathname component before
+> the trailing <slash> characters names an existing directory or a
+> directory entry that is to be created for a directory immediately
+> after the pathname is resolved.
+
+The Linux pathname walking code (mostly in `fs/namei.c`) deals with
+all of these issues: breaking the path into components, handling the
+"everything else" quite separately from the final component, and
+checking that the trailing slash is not used where it isn't
+permitted. It also addresses the important issue of concurrent
+access.
+
+While one process is looking up a pathname, another might be making
+changes that affect that lookup. One fairly extreme case is that if
+"a/b" were renamed to "a/c/b" while another process were looking up
+"a/b/..", that process might successfully resolve on "a/c".
+Most races are much more subtle, and a big part of the task of
+pathname lookup is to prevent them from having damaging effects. Many
+of the possible races are seen most clearly in the context of the
+"dcache" and an understanding of that is central to understanding
+pathname lookup.
+
+More than just a cache.
+-----------------------
+
+The "dcache" caches information about names in each filesystem to
+make them quickly available for lookup. Each entry (known as a
+"dentry") contains three significant fields: a component name, a
+pointer to a parent dentry, and a pointer to the "inode" which
+contains further information about the object in that parent with
+the given name. The inode pointer can be `NULL` indicating that the
+name doesn't exist in the parent. While there can be linkage in the
+dentry of a directory to the dentries of the children, that linkage is
+not used for pathname lookup, and so will not be considered here.
+
+The dcache has a number of uses apart from accelerating lookup. One
+that will be particularly relevant is that it is closely integrated
+with the mount table that records which filesystem is mounted where.
+What the mount table actually stores is which dentry is mounted on top
+of which other dentry.
+
+When considering the dcache, we have another of our "two types"
+distinctions: there are two types of filesystems.
+
+Some filesystems ensure that the information in the dcache is always
+completely accurate (though not necessarily complete). This can allow
+the VFS to determine if a particular file does or doesn't exist
+without checking with the filesystem, and means that the VFS can
+protect the filesystem against certain races and other problems.
+These are typically "local" filesystems such as ext3, XFS, and Btrfs.
+
+Other filesystems don't provide that guarantee because they cannot.
+These are typically filesystems that are shared across a network,
+whether remote filesystems like NFS and 9P, or cluster filesystems
+like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
+cached information, and must provide their own protection against
+awkward races. The VFS can detect these filesystems by the
+`DCACHE_OP_REVALIDATE` flag being set in the dentry.
+
+REF-walk: simple concurrency management with refcounts and spinlocks
+--------------------------------------------------------------------
+
+With all of those divisions carefully classified, we can now start
+looking at the actual process of walking along a path. In particular
+we will start with the handling of the "everything else" part of a
+pathname, and focus on the "REF-walk" approach to concurrency
+management. This code is found in the `link_path_walk()` function, if
+you ignore all the places that only run when "`LOOKUP_RCU`"
+(indicating the use of RCU-walk) is set.
+
+[Meet the Lockers]: https://lwn.net/Articles/453685/
+
+REF-walk is fairly heavy-handed with locks and reference counts. Not
+as heavy-handed as in the old "big kernel lock" days, but certainly not
+afraid of taking a lock when one is needed. It uses a variety of
+different concurrency controls. A background understanding of the
+various primitives is assumed, or can be gleaned from elsewhere such
+as in [Meet the Lockers].
+
+The locking mechanisms used by REF-walk include:
+
+### dentry->d_lockref ###
+
+This uses the lockref primitive to provide both a spinlock and a
+reference count. The special-sauce of this primitive is that the
+conceptual sequence "lock; inc_ref; unlock;" can often be performed
+with a single atomic memory operation.
+
+Holding a reference on a dentry ensures that the dentry won't suddenly
+be freed and used for something else, so the values in various fields
+will behave as expected. It also protects the `->d_inode` reference
+to the inode to some extent.
+
+The association between a dentry and its inode is fairly permanent.
+For example, when a file is renamed, the dentry and inode move
+together to the new location. When a file is created the dentry will
+initially be negative (i.e. `d_inode` is `NULL`), and will be assigned
+to the new inode as part of the act of creation.
+
+When a file is deleted, this can be reflected in the cache either by
+setting `d_inode` to `NULL`, or by removing it from the hash table
+(described shortly) used to look up the name in the parent directory.
+If the dentry is still in use the second option is used as it is
+perfectly legal to keep using an open file after it has been deleted
+and having the dentry around helps. If the dentry is not otherwise in
+use (i.e. if the refcount in `d_lockref` is one), only then will
+`d_inode` be set to `NULL`. Doing it this way is more efficient for a
+very common case.
+
+So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode`
+value will never be changed.
+
+### dentry->d_lock ###
+
+`d_lock` is a synonym for the spinlock that is part of `d_lockref` above.
+For our purposes, holding this lock protects against the dentry being
+renamed or unlinked. In particular, its parent (`d_parent`), and its
+name (`d_name`) cannot be changed, and it cannot be removed from the
+dentry hash table.
+
+When looking for a name in a directory, REF-walk takes `d_lock` on
+each candidate dentry that it finds in the hash table and then checks
+that the parent and name are correct. So it doesn't lock the parent
+while searching in the cache; it only locks children.
+
+When looking for the parent for a given name (to handle "`..`"),
+REF-walk can take `d_lock` to get a stable reference to `d_parent`,
+but it first tries a more lightweight approach. As seen in
+`dget_parent()`, if a reference can be claimed on the parent, and if
+subsequently `d_parent` can be seen to have not changed, then there is
+no need to actually take the lock on the child.
+
+### rename_lock ###
+
+Looking up a given name in a given directory involves computing a hash
+from the two values (the name and the dentry of the directory),
+accessing that slot in a hash table, and searching the linked list
+that is found there.
+
+When a dentry is renamed, the name and the parent dentry can both
+change so the hash will almost certainly change too. This would move the
+dentry to a different chain in the hash table. If a filename search
+happened to be looking at a dentry that was moved in this way,
+it might end up continuing the search down the wrong chain,
+and so miss out on part of the correct chain.
+
+The name-lookup process (`d_lookup()`) does _not_ try to prevent this
+from happening, but only to detect when it happens.
+`rename_lock` is a seqlock that is updated whenever any dentry is
+renamed. If `d_lookup` finds that a rename happened while it
+unsuccessfully scanned a chain in the hash table, it simply tries
+again.
+
+### inode->i_mutex ###
+
+`i_mutex` is a mutex that serializes all changes to a particular
+directory. This ensures that, for example, an `unlink()` and a `rename()`
+cannot both happen at the same time. It also keeps the directory
+stable while the filesystem is asked to look up a name that is not
+currently in the dcache.
+
+This has a complementary role to that of `d_lock`: `i_mutex` on a
+directory protects all of the names in that directory, while `d_lock`
+on a name protects just one name in a directory. Most changes to the
+dcache hold `i_mutex` on the relevant directory inode and briefly take
+`d_lock` on one or more the dentries while the change happens. One
+exception is when idle dentries are removed from the dcache due to
+memory pressure. This uses `d_lock`, but `i_mutex` plays no role.
+
+The mutex affects pathname lookup in two distinct ways. Firstly it
+serializes lookup of a name in a directory. `walk_component()` uses
+`lookup_fast()` first which, in turn, checks to see if the name is in the cache,
+using only `d_lock` locking. If the name isn't found, then `walk_component()`
+falls back to `lookup_slow()` which takes `i_mutex`, checks again that
+the name isn't in the cache, and then calls in to the filesystem to get a
+definitive answer. A new dentry will be added to the cache regardless of
+the result.
+
+Secondly, when pathname lookup reaches the final component, it will
+sometimes need to take `i_mutex` before performing the last lookup so
+that the required exclusion can be achieved. How path lookup chooses
+to take, or not take, `i_mutex` is one of the
+issues addressed in a subsequent section.
+
+### mnt->mnt_count ###
+
+`mnt_count` is a per-CPU reference counter on "`mount`" structures.
+Per-CPU here means that incrementing the count is cheap as it only
+uses CPU-local memory, but checking if the count is zero is expensive as
+it needs to check with every CPU. Taking a `mnt_count` reference
+prevents the mount structure from disappearing as the result of regular
+unmount operations, but does not prevent a "lazy" unmount. So holding
+`mnt_count` doesn't ensure that the mount remains in the namespace and,
+in particular, doesn't stabilize the link to the mounted-on dentry. It
+does, however, ensure that the `mount` data structure remains coherent,
+and it provides a reference to the root dentry of the mounted
+filesystem. So a reference through `->mnt_count` provides a stable
+reference to the mounted dentry, but not the mounted-on dentry.
+
+### mount_lock ###
+
+`mount_lock` is a global seqlock, a bit like `rename_lock`. It can be used to
+check if any change has been made to any mount points.
+
+While walking down the tree (away from the root) this lock is used when
+crossing a mount point to check that the crossing was safe. That is,
+the value in the seqlock is read, then the code finds the mount that
+is mounted on the current directory, if there is one, and increments
+the `mnt_count`. Finally the value in `mount_lock` is checked against
+the old value. If there is no change, then the crossing was safe. If there
+was a change, the `mnt_count` is decremented and the whole process is
+retried.
+
+When walking up the tree (towards the root) by following a ".." link,
+a little more care is needed. In this case the seqlock (which
+contains both a counter and a spinlock) is fully locked to prevent
+any changes to any mount points while stepping up. This locking is
+needed to stabilize the link to the mounted-on dentry, which the
+refcount on the mount itself doesn't ensure.
+
+### RCU ###
+
+Finally the global (but extremely lightweight) RCU read lock is held
+from time to time to ensure certain data structures don't get freed
+unexpectedly.
+
+In particular it is held while scanning chains in the dcache hash
+table, and the mount point hash table.
+
+Bringing it together with `struct nameidata`
+--------------------------------------------
+
+[First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
+
+Throughout the process of walking a path, the current status is stored
+in a `struct nameidata`, "namei" being the traditional name - dating
+all the way back to [First Edition Unix] - of the function that
+converts a "name" to an "inode". `struct nameidata` contains (among
+other fields):
+
+### `struct path path` ###
+
+A `path` contains a `struct vfsmount` (which is
+embedded in a `struct mount`) and a `struct dentry`. Together these
+record the current status of the walk. They start out referring to the
+starting point (the current working directory, the root directory, or some other
+directory identified by a file descriptor), and are updated on each
+step. A reference through `d_lockref` and `mnt_count` is always
+held.
+
+### `struct qstr last` ###
+
+This is a string together with a length (i.e. _not_ `nul` terminated)
+that is the "next" component in the pathname.
+
+### `int last_type` ###
+
+This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or
+`LAST_BIND`. The `last` field is only valid if the type is
+`LAST_NORM`. `LAST_BIND` is used when following a symlink and no
+components of the symlink have been processed yet. Others should be
+fairly self-explanatory.
+
+### `struct path root` ###
+
+This is used to hold a reference to the effective root of the
+filesystem. Often that reference won't be needed, so this field is
+only assigned the first time it is used, or when a non-standard root
+is requested. Keeping a reference in the `nameidata` ensures that
+only one root is in effect for the entire path walk, even if it races
+with a `chroot()` system call.
+
+The root is needed when either of two conditions holds: (1) either the
+pathname or a symbolic link starts with a "'/'", or (2) a "`..`"
+component is being handled, since "`..`" from the root must always stay
+at the root. The value used is usually the current root directory of
+the calling process. An alternate root can be provided as when
+`sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call
+`mount_subtree()`. In each case a pathname is being looked up in a very
+specific part of the filesystem, and the lookup must not be allowed to
+escape that subtree. It works a bit like a local `chroot()`.
+
+Ignoring the handling of symbolic links, we can now describe the
+"`link_path_walk()`" function, which handles the lookup of everything
+except the final component as:
+
+> Given a path (`name`) and a nameidata structure (`nd`), check that the
+> current directory has execute permission and then advance `name`
+> over one component while updating `last_type` and `last`. If that
+> was the final component, then return, otherwise call
+> `walk_component()` and repeat from the top.
+
+`walk_component()` is even easier. If the component is `LAST_DOTS`,
+it calls `handle_dots()` which does the necessary locking as already
+described. If it finds a `LAST_NORM` component it first calls
+"`lookup_fast()`" which only looks in the dcache, but will ask the
+filesystem to revalidate the result if it is that sort of filesystem.
+If that doesn't get a good result, it calls "`lookup_slow()`" which
+takes the `i_mutex`, rechecks the cache, and then asks the filesystem
+to find a definitive answer. Each of these will call
+`follow_managed()` (as described below) to handle any mount points.
+
+In the absence of symbolic links, `walk_component()` creates a new
+`struct path` containing a counted reference to the new dentry and a
+reference to the new `vfsmount` which is only counted if it is
+different from the previous `vfsmount`. It then calls
+`path_to_nameidata()` to install the new `struct path` in the
+`struct nameidata` and drop the unneeded references.
+
+This "hand-over-hand" sequencing of getting a reference to the new
+dentry before dropping the reference to the previous dentry may
+seem obvious, but is worth pointing out so that we will recognize its
+analogue in the "RCU-walk" version.
+
+Handling the final component.
+-----------------------------
+
+`link_path_walk()` only walks as far as setting `nd->last` and
+`nd->last_type` to refer to the final component of the path. It does
+not call `walk_component()` that last time. Handling that final
+component remains for the caller to sort out. Those callers are
+`path_lookupat()`, `path_parentat()`, `path_mountpoint()` and
+`path_openat()` each of which handles the differing requirements of
+different system calls.
+
+`path_parentat()` is clearly the simplest - it just wraps a little bit
+of housekeeping around `link_path_walk()` and returns the parent
+directory and final component to the caller. The caller will be either
+aiming to create a name (via `filename_create()`) or remove or rename
+a name (in which case `user_path_parent()` is used). They will use
+`i_mutex` to exclude other changes while they validate and then
+perform their operation.
+
+`path_lookupat()` is nearly as simple - it is used when an existing
+object is wanted such as by `stat()` or `chmod()`. It essentially just
+calls `walk_component()` on the final component through a call to
+`lookup_last()`. `path_lookupat()` returns just the final dentry.
+
+`path_mountpoint()` handles the special case of unmounting which must
+not try to revalidate the mounted filesystem. It effectively
+contains, through a call to `mountpoint_last()`, an alternate
+implementation of `lookup_slow()` which skips that step. This is
+important when unmounting a filesystem that is inaccessible, such as
+one provided by a dead NFS server.
+
+Finally `path_openat()` is used for the `open()` system call; it
+contains, in support functions starting with "`do_last()`", all the
+complexity needed to handle the different subtleties of O_CREAT (with
+or without O_EXCL), final "`/`" characters, and trailing symbolic
+links. We will revisit this in the final part of this series, which
+focuses on those symbolic links. "`do_last()`" will sometimes, but
+not always, take `i_mutex`, depending on what it finds.
+
+Each of these, or the functions which call them, need to be alert to
+the possibility that the final component is not `LAST_NORM`. If the
+goal of the lookup is to create something, then any value for
+`last_type` other than `LAST_NORM` will result in an error. For
+example if `path_parentat()` reports `LAST_DOTDOT`, then the caller
+won't try to create that name. They also check for trailing slashes
+by testing `last.name[last.len]`. If there is any character beyond
+the final component, it must be a trailing slash.
+
+Revalidation and automounts
+---------------------------
+
+Apart from symbolic links, there are only two parts of the "REF-walk"
+process not yet covered. One is the handling of stale cache entries
+and the other is automounts.
+
+On filesystems that require it, the lookup routines will call the
+`->d_revalidate()` dentry method to ensure that the cached information
+is current. This will often confirm validity or update a few details
+from a server. In some cases it may find that there has been change
+further up the path and that something that was thought to be valid
+previously isn't really. When this happens the lookup of the whole
+path is aborted and retried with the "`LOOKUP_REVAL`" flag set. This
+forces revalidation to be more thorough. We will see more details of
+this retry process in the next article.
+
+Automount points are locations in the filesystem where an attempt to
+lookup a name can trigger changes to how that lookup should be
+handled, in particular by mounting a filesystem there. These are
+covered in greater detail in autofs4.txt in the Linux documentation
+tree, but a few notes specifically related to path lookup are in order
+here.
+
+The Linux VFS has a concept of "managed" dentries which is reflected
+in function names such as "`follow_managed()`". There are three
+potentially interesting things about these dentries corresponding
+to three different flags that might be set in `dentry->d_flags`:
+
+### `DCACHE_MANAGE_TRANSIT` ###
+
+If this flag has been set, then the filesystem has requested that the
+`d_manage()` dentry operation be called before handling any possible
+mount point. This can perform two particular services:
+
+It can block to avoid races. If an automount point is being
+unmounted, the `d_manage()` function will usually wait for that
+process to complete before letting the new lookup proceed and possibly
+trigger a new automount.
+
+It can selectively allow only some processes to transit through a
+mount point. When a server process is managing automounts, it may
+need to access a directory without triggering normal automount
+processing. That server process can identify itself to the `autofs`
+filesystem, which will then give it a special pass through
+`d_manage()` by returning `-EISDIR`.
+
+### `DCACHE_MOUNTED` ###
+
+This flag is set on every dentry that is mounted on. As Linux
+supports multiple filesystem namespaces, it is possible that the
+dentry may not be mounted on in *this* namespace, just in some
+other. So this flag is seen as a hint, not a promise.
+
+If this flag is set, and `d_manage()` didn't return `-EISDIR`,
+`lookup_mnt()` is called to examine the mount hash table (honoring the
+`mount_lock` described earlier) and possibly return a new `vfsmount`
+and a new `dentry` (both with counted references).
+
+### `DCACHE_NEED_AUTOMOUNT` ###
+
+If `d_manage()` allowed us to get this far, and `lookup_mnt()` didn't
+find a mount point, then this flag causes the `d_automount()` dentry
+operation to be called.
+
+The `d_automount()` operation can be arbitrarily complex and may
+communicate with server processes etc. but it should ultimately either
+report that there was an error, that there was nothing to mount, or
+should provide an updated `struct path` with new `dentry` and `vfsmount`.
+
+In the latter case, `finish_automount()` will be called to safely
+install the new mount point into the mount table.
+
+There is no new locking of import here and it is important that no
+locks (only counted references) are held over this processing due to
+the very real possibility of extended delays.
+This will become more important next time when we examine RCU-walk
+which is particularly sensitive to delays.
+
+RCU-walk - faster pathname lookup in Linux
+==========================================
+
+RCU-walk is another algorithm for performing pathname lookup in Linux.
+It is in many ways similar to REF-walk and the two share quite a bit
+of code. The significant difference in RCU-walk is how it allows for
+the possibility of concurrent access.
+
+We noted that REF-walk is complex because there are numerous details
+and special cases. RCU-walk reduces this complexity by simply
+refusing to handle a number of cases -- it instead falls back to
+REF-walk. The difficulty with RCU-walk comes from a different
+direction: unfamiliarity. The locking rules when depending on RCU are
+quite different from traditional locking, so we will spend a little extra
+time when we come to those.
+
+Clear demarcation of roles
+--------------------------
+
+The easiest way to manage concurrency is to forcibly stop any other
+thread from changing the data structures that a given thread is
+looking at. In cases where no other thread would even think of
+changing the data and lots of different threads want to read at the
+same time, this can be very costly. Even when using locks that permit
+multiple concurrent readers, the simple act of updating the count of
+the number of current readers can impose an unwanted cost. So the
+goal when reading a shared data structure that no other process is
+changing is to avoid writing anything to memory at all. Take no
+locks, increment no counts, leave no footprints.
+
+The REF-walk mechanism already described certainly doesn't follow this
+principle, but then it is really designed to work when there may well
+be other threads modifying the data. RCU-walk, in contrast, is
+designed for the common situation where there are lots of frequent
+readers and only occasional writers. This may not be common in all
+parts of the filesystem tree, but in many parts it will be. For the
+other parts it is important that RCU-walk can quickly fall back to
+using REF-walk.
+
+Pathname lookup always starts in RCU-walk mode but only remains there
+as long as what it is looking for is in the cache and is stable. It
+dances lightly down the cached filesystem image, leaving no footprints
+and carefully watching where it is, to be sure it doesn't trip. If it
+notices that something has changed or is changing, or if something
+isn't in the cache, then it tries to stop gracefully and switch to
+REF-walk.
+
+This stopping requires getting a counted reference on the current
+`vfsmount` and `dentry`, and ensuring that these are still valid -
+that a path walk with REF-walk would have found the same entries.
+This is an invariant that RCU-walk must guarantee. It can only make
+decisions, such as selecting the next step, that are decisions which
+REF-walk could also have made if it were walking down the tree at the
+same time. If the graceful stop succeeds, the rest of the path is
+processed with the reliable, if slightly sluggish, REF-walk. If
+RCU-walk finds it cannot stop gracefully, it simply gives up and
+restarts from the top with REF-walk.
+
+This pattern of "try RCU-walk, if that fails try REF-walk" can be
+clearly seen in functions like `filename_lookup()`,
+`filename_parentat()`, `filename_mountpoint()`,
+`do_filp_open()`, and `do_file_open_root()`. These five
+correspond roughly to the four `path_`* functions we met earlier,
+each of which calls `link_path_walk()`. The `path_*` functions are
+called using different mode flags until a mode is found which works.
+They are first called with `LOOKUP_RCU` set to request "RCU-walk". If
+that fails with the error `ECHILD` they are called again with no
+special flag to request "REF-walk". If either of those report the
+error `ESTALE` a final attempt is made with `LOOKUP_REVAL` set (and no
+`LOOKUP_RCU`) to ensure that entries found in the cache are forcibly
+revalidated - normally entries are only revalidated if the filesystem
+determines that they are too old to trust.
+
+The `LOOKUP_RCU` attempt may drop that flag internally and switch to
+REF-walk, but will never then try to switch back to RCU-walk. Places
+that trip up RCU-walk are much more likely to be near the leaves and
+so it is very unlikely that there will be much, if any, benefit from
+switching back.
+
+RCU and seqlocks: fast and light
+--------------------------------
+
+RCU is, unsurprisingly, critical to RCU-walk mode. The
+`rcu_read_lock()` is held for the entire time that RCU-walk is walking
+down a path. The particular guarantee it provides is that the key
+data structures - dentries, inodes, super_blocks, and mounts - will
+not be freed while the lock is held. They might be unlinked or
+invalidated in one way or another, but the memory will not be
+repurposed so values in various fields will still be meaningful. This
+is the only guarantee that RCU provides; everything else is done using
+seqlocks.
+
+As we saw above, REF-walk holds a counted reference to the current
+dentry and the current vfsmount, and does not release those references
+before taking references to the "next" dentry or vfsmount. It also
+sometimes takes the `d_lock` spinlock. These references and locks are
+taken to prevent certain changes from happening. RCU-walk must not
+take those references or locks and so cannot prevent such changes.
+Instead, it checks to see if a change has been made, and aborts or
+retries if it has.
+
+To preserve the invariant mentioned above (that RCU-walk may only make
+decisions that REF-walk could have made), it must make the checks at
+or near the same places that REF-walk holds the references. So, when
+REF-walk increments a reference count or takes a spinlock, RCU-walk
+samples the status of a seqlock using `read_seqcount_begin()` or a
+similar function. When REF-walk decrements the count or drops the
+lock, RCU-walk checks if the sampled status is still valid using
+`read_seqcount_retry()` or similar.
+
+However, there is a little bit more to seqlocks than that. If
+RCU-walk accesses two different fields in a seqlock-protected
+structure, or accesses the same field twice, there is no a priori
+guarantee of any consistency between those accesses. When consistency
+is needed - which it usually is - RCU-walk must take a copy and then
+use `read_seqcount_retry()` to validate that copy.
+
+`read_seqcount_retry()` not only checks the sequence number, but also
+imposes a memory barrier so that no memory-read instruction from
+*before* the call can be delayed until *after* the call, either by the
+CPU or by the compiler. A simple example of this can be seen in
+`slow_dentry_cmp()` which, for filesystems which do not use simple
+byte-wise name equality, calls into the filesystem to compare a name
+against a dentry. The length and name pointer are copied into local
+variables, then `read_seqcount_retry()` is called to confirm the two
+are consistent, and only then is `->d_compare()` called. When
+standard filename comparison is used, `dentry_cmp()` is called
+instead. Notably it does _not_ use `read_seqcount_retry()`, but
+instead has a large comment explaining why the consistency guarantee
+isn't necessary. A subsequent `read_seqcount_retry()` will be
+sufficient to catch any problem that could occur at this point.
+
+With that little refresher on seqlocks out of the way we can look at
+the bigger picture of how RCU-walk uses seqlocks.
+
+### `mount_lock` and `nd->m_seq` ###
+
+We already met the `mount_lock` seqlock when REF-walk used it to
+ensure that crossing a mount point is performed safely. RCU-walk uses
+it for that too, but for quite a bit more.
+
+Instead of taking a counted reference to each `vfsmount` as it
+descends the tree, RCU-walk samples the state of `mount_lock` at the
+start of the walk and stores this initial sequence number in the
+`struct nameidata` in the `m_seq` field. This one lock and one
+sequence number are used to validate all accesses to all `vfsmounts`,
+and all mount point crossings. As changes to the mount table are
+relatively rare, it is reasonable to fall back on REF-walk any time
+that any "mount" or "unmount" happens.
+
+`m_seq` is checked (using `read_seqretry()`) at the end of an RCU-walk
+sequence, whether switching to REF-walk for the rest of the path or
+when the end of the path is reached. It is also checked when stepping
+down over a mount point (in `__follow_mount_rcu()`) or up (in
+`follow_dotdot_rcu()`). If it is ever found to have changed, the
+whole RCU-walk sequence is aborted and the path is processed again by
+REF-walk.
+
+If RCU-walk finds that `mount_lock` hasn't changed then it can be sure
+that, had REF-walk taken counted references on each vfsmount, the
+results would have been the same. This ensures the invariant holds,
+at least for vfsmount structures.
+
+### `dentry->d_seq` and `nd->seq`. ###
+
+In place of taking a count or lock on `d_reflock`, RCU-walk samples
+the per-dentry `d_seq` seqlock, and stores the sequence number in the
+`seq` field of the nameidata structure, so `nd->seq` should always be
+the current sequence number of `nd->dentry`. This number needs to be
+revalidated after copying, and before using, the name, parent, or
+inode of the dentry.
+
+The handling of the name we have already looked at, and the parent is
+only accessed in `follow_dotdot_rcu()` which fairly trivially follows
+the required pattern, though it does so for three different cases.
+
+When not at a mount point, `d_parent` is followed and its `d_seq` is
+collected. When we are at a mount point, we instead follow the
+`mnt->mnt_mountpoint` link to get a new dentry and collect its
+`d_seq`. Then, after finally finding a `d_parent` to follow, we must
+check if we have landed on a mount point and, if so, must find that
+mount point and follow the `mnt->mnt_root` link. This would imply a
+somewhat unusual, but certainly possible, circumstance where the
+starting point of the path lookup was in part of the filesystem that
+was mounted on, and so not visible from the root.
+
+The inode pointer, stored in `->d_inode`, is a little more
+interesting. The inode will always need to be accessed at least
+twice, once to determine if it is NULL and once to verify access
+permissions. Symlink handling requires a validated inode pointer too.
+Rather than revalidating on each access, a copy is made on the first
+access and it is stored in the `inode` field of `nameidata` from where
+it can be safely accessed without further validation.
+
+`lookup_fast()` is the only lookup routine that is used in RCU-mode,
+`lookup_slow()` being too slow and requiring locks. It is in
+`lookup_fast()` that we find the important "hand over hand" tracking
+of the current dentry.
+
+The current `dentry` and current `seq` number are passed to
+`__d_lookup_rcu()` which, on success, returns a new `dentry` and a
+new `seq` number. `lookup_fast()` then copies the inode pointer and
+revalidates the new `seq` number. It then validates the old `dentry`
+with the old `seq` number one last time and only then continues. This
+process of getting the `seq` number of the new dentry and then
+checking the `seq` number of the old exactly mirrors the process of
+getting a counted reference to the new dentry before dropping that for
+the old dentry which we saw in REF-walk.
+
+### No `inode->i_mutex` or even `rename_lock` ###
+
+A mutex is a fairly heavyweight lock that can only be taken when it is
+permissible to sleep. As `rcu_read_lock()` forbids sleeping,
+`inode->i_mutex` plays no role in RCU-walk. If some other thread does
+take `i_mutex` and modifies the directory in a way that RCU-walk needs
+to notice, the result will be either that RCU-walk fails to find the
+dentry that it is looking for, or it will find a dentry which
+`read_seqretry()` won't validate. In either case it will drop down to
+REF-walk mode which can take whatever locks are needed.
+
+Though `rename_lock` could be used by RCU-walk as it doesn't require
+any sleeping, RCU-walk doesn't bother. REF-walk uses `rename_lock` to
+protect against the possibility of hash chains in the dcache changing
+while they are being searched. This can result in failing to find
+something that actually is there. When RCU-walk fails to find
+something in the dentry cache, whether it is really there or not, it
+already drops down to REF-walk and tries again with appropriate
+locking. This neatly handles all cases, so adding extra checks on
+rename_lock would bring no significant value.
+
+`unlazy walk()` and `complete_walk()`
+-------------------------------------
+
+That "dropping down to REF-walk" typically involves a call to
+`unlazy_walk()`, so named because "RCU-walk" is also sometimes
+referred to as "lazy walk". `unlazy_walk()` is called when
+following the path down to the current vfsmount/dentry pair seems to
+have proceeded successfully, but the next step is problematic. This
+can happen if the next name cannot be found in the dcache, if
+permission checking or name revalidation couldn't be achieved while
+the `rcu_read_lock()` is held (which forbids sleeping), if an
+automount point is found, or in a couple of cases involving symlinks.
+It is also called from `complete_walk()` when the lookup has reached
+the final component, or the very end of the path, depending on which
+particular flavor of lookup is used.
+
+Other reasons for dropping out of RCU-walk that do not trigger a call
+to `unlazy_walk()` are when some inconsistency is found that cannot be
+handled immediately, such as `mount_lock` or one of the `d_seq`
+seqlocks reporting a change. In these cases the relevant function
+will return `-ECHILD` which will percolate up until it triggers a new
+attempt from the top using REF-walk.
+
+For those cases where `unlazy_walk()` is an option, it essentially
+takes a reference on each of the pointers that it holds (vfsmount,
+dentry, and possibly some symbolic links) and then verifies that the
+relevant seqlocks have not been changed. If there have been changes,
+it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk
+has been a success and the lookup process continues.
+
+Taking a reference on those pointers is not quite as simple as just
+incrementing a counter. That works to take a second reference if you
+already have one (often indirectly through another object), but it
+isn't sufficient if you don't actually have a counted reference at
+all. For `dentry->d_lockref`, it is safe to increment the reference
+counter to get a reference unless it has been explicitly marked as
+"dead" which involves setting the counter to `-128`.
+`lockref_get_not_dead()` achieves this.
+
+For `mnt->mnt_count` it is safe to take a reference as long as
+`mount_lock` is then used to validate the reference. If that
+validation fails, it may *not* be safe to just drop that reference in
+the standard way of calling `mnt_put()` - an unmount may have
+progressed too far. So the code in `legitimize_mnt()`, when it
+finds that the reference it got might not be safe, checks the
+`MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is
+correct, or if it should just decrement the count and pretend none of
+this ever happened.
+
+Taking care in filesystems
+---------------------------
+
+RCU-walk depends almost entirely on cached information and often will
+not call into the filesystem at all. However there are two places,
+besides the already-mentioned component-name comparison, where the
+file system might be included in RCU-walk, and it must know to be
+careful.
+
+If the filesystem has non-standard permission-checking requirements -
+such as a networked filesystem which may need to check with the server
+- the `i_op->permission` interface might be called during RCU-walk.
+In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it
+knows not to sleep, but to return `-ECHILD` if it cannot complete
+promptly. `i_op->permission` is given the inode pointer, not the
+dentry, so it doesn't need to worry about further consistency checks.
+However if it accesses any other filesystem data structures, it must
+ensure they are safe to be accessed with only the `rcu_read_lock()`
+held. This typically means they must be freed using `kfree_rcu()` or
+similar.
+
+[`READ_ONCE()`]: https://lwn.net/Articles/624126/
+
+If the filesystem may need to revalidate dcache entries, then
+`d_op->d_revalidate` may be called in RCU-walk too. This interface
+*is* passed the dentry but does not have access to the `inode` or the
+`seq` number from the `nameidata`, so it needs to be extra careful
+when accessing fields in the dentry. This "extra care" typically
+involves using `ACCESS_ONCE()` or the newer [`READ_ONCE()`] to access
+fields, and verifying the result is not NULL before using it. This
+pattern can be see in `nfs_lookup_revalidate()`.
+
+A pair of patterns
+------------------
+
+In various places in the details of REF-walk and RCU-walk, and also in
+the big picture, there are a couple of related patterns that are worth
+being aware of.
+
+The first is "try quickly and check, if that fails try slowly". We
+can see that in the high-level approach of first trying RCU-walk and
+then trying REF-walk, and in places where `unlazy_walk()` is used to
+switch to REF-walk for the rest of the path. We also saw it earlier
+in `dget_parent()` when following a "`..`" link. It tries a quick way
+to get a reference, then falls back to taking locks if needed.
+
+The second pattern is "try quickly and check, if that fails try
+again - repeatedly". This is seen with the use of `rename_lock` and
+`mount_lock` in REF-walk. RCU-walk doesn't make use of this pattern -
+if anything goes wrong it is much safer to just abort and try a more
+sedate approach.
+
+The emphasis here is "try quickly and check". It should probably be
+"try quickly _and carefully,_ then check". The fact that checking is
+needed is a reminder that the system is dynamic and only a limited
+number of things are safe at all. The most likely cause of errors in
+this whole process is assuming something is safe when in reality it
+isn't. Careful consideration of what exactly guarantees the safety of
+each access is sometimes necessary.
+
+A walk among the symlinks
+=========================
+
+There are several basic issues that we will examine to understand the
+handling of symbolic links: the symlink stack, together with cache
+lifetimes, will help us understand the overall recursive handling of
+symlinks and lead to the special care needed for the final component.
+Then a consideration of access-time updates and summary of the various
+flags controlling lookup will finish the story.
+
+The symlink stack
+-----------------
+
+There are only two sorts of filesystem objects that can usefully
+appear in a path prior to the final component: directories and symlinks.
+Handling directories is quite straightforward: the new directory
+simply becomes the starting point at which to interpret the next
+component on the path. Handling symbolic links requires a bit more
+work.
+
+Conceptually, symbolic links could be handled by editing the path. If
+a component name refers to a symbolic link, then that component is
+replaced by the body of the link and, if that body starts with a '/',
+then all preceding parts of the path are discarded. This is what the
+"`readlink -f`" command does, though it also edits out "`.`" and
+"`..`" components.
+
+Directly editing the path string is not really necessary when looking
+up a path, and discarding early components is pointless as they aren't
+looked at anyway. Keeping track of all remaining components is
+important, but they can of course be kept separately; there is no need
+to concatenate them. As one symlink may easily refer to another,
+which in turn can refer to a third, we may need to keep the remaining
+components of several paths, each to be processed when the preceding
+ones are completed. These path remnants are kept on a stack of
+limited size.
+
+There are two reasons for placing limits on how many symlinks can
+occur in a single path lookup. The most obvious is to avoid loops.
+If a symlink referred to itself either directly or through
+intermediaries, then following the symlink can never complete
+successfully - the error `ELOOP` must be returned. Loops can be
+detected without imposing limits, but limits are the simplest solution
+and, given the second reason for restriction, quite sufficient.
+
+[outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
+
+The second reason was [outlined recently] by Linus:
+
+> Because it's a latency and DoS issue too. We need to react well to
+> true loops, but also to "very deep" non-loops. It's not about memory
+> use, it's about users triggering unreasonable CPU resources.
+
+Linux imposes a limit on the length of any pathname: `PATH_MAX`, which
+is 4096. There are a number of reasons for this limit; not letting the
+kernel spend too much time on just one path is one of them. With
+symbolic links you can effectively generate much longer paths so some
+sort of limit is needed for the same reason. Linux imposes a limit of
+at most 40 symlinks in any one path lookup. It previously imposed a
+further limit of eight on the maximum depth of recursion, but that was
+raised to 40 when a separate stack was implemented, so there is now
+just the one limit.
+
+The `nameidata` structure that we met in an earlier article contains a
+small stack that can be used to store the remaining part of up to two
+symlinks. In many cases this will be sufficient. If it isn't, a
+separate stack is allocated with room for 40 symlinks. Pathname
+lookup will never exceed that stack as, once the 40th symlink is
+detected, an error is returned.
+
+It might seem that the name remnants are all that needs to be stored on
+this stack, but we need a bit more. To see that, we need to move on to
+cache lifetimes.
+
+Storage and lifetime of cached symlinks
+---------------------------------------
+
+Like other filesystem resources, such as inodes and directory
+entries, symlinks are cached by Linux to avoid repeated costly access
+to external storage. It is particularly important for RCU-walk to be
+able to find and temporarily hold onto these cached entries, so that
+it doesn't need to drop down into REF-walk.
+
+[object-oriented design pattern]: https://lwn.net/Articles/446317/
+
+While each filesystem is free to make its own choice, symlinks are
+typically stored in one of two places. Short symlinks are often
+stored directly in the inode. When a filesystem allocates a `struct
+inode` it typically allocates extra space to store private data (a
+common [object-oriented design pattern] in the kernel). This will
+sometimes include space for a symlink. The other common location is
+in the page cache, which normally stores the content of files. The
+pathname in a symlink can be seen as the content of that symlink and
+can easily be stored in the page cache just like file content.
+
+When neither of these is suitable, the next most likely scenario is
+that the filesystem will allocate some temporary memory and copy or
+construct the symlink content into that memory whenever it is needed.
+
+When the symlink is stored in the inode, it has the same lifetime as
+the inode which, itself, is protected by RCU or by a counted reference
+on the dentry. This means that the mechanisms that pathname lookup
+uses to access the dcache and icache (inode cache) safely are quite
+sufficient for accessing some cached symlinks safely. In these cases,
+the `i_link` pointer in the inode is set to point to wherever the
+symlink is stored and it can be accessed directly whenever needed.
+
+When the symlink is stored in the page cache or elsewhere, the
+situation is not so straightforward. A reference on a dentry or even
+on an inode does not imply any reference on cached pages of that
+inode, and even an `rcu_read_lock()` is not sufficient to ensure that
+a page will not disappear. So for these symlinks the pathname lookup
+code needs to ask the filesystem to provide a stable reference and,
+significantly, needs to release that reference when it is finished
+with it.
+
+Taking a reference to a cache page is often possible even in RCU-walk
+mode. It does require making changes to memory, which is best avoided,
+but that isn't necessarily a big cost and it is better than dropping
+out of RCU-walk mode completely. Even filesystems that allocate
+space to copy the symlink into can use `GFP_ATOMIC` to often successfully
+allocate memory without the need to drop out of RCU-walk. If a
+filesystem cannot successfully get a reference in RCU-walk mode, it
+must return `-ECHILD` and `unlazy_walk()` will be called to return to
+REF-walk mode in which the filesystem is allowed to sleep.
+
+The place for all this to happen is the `i_op->follow_link()` inode
+method. In the present mainline code this is never actually called in
+RCU-walk mode as the rewrite is not quite complete. It is likely that
+in a future release this method will be passed an `inode` pointer when
+called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
+validated pointer. Much like the `i_op->permission()` method we
+looked at previously, `->follow_link()` would need to be careful that
+all the data structures it references are safe to be accessed while
+holding no counted reference, only the RCU lock. Though getting a
+reference with `->follow_link()` is not yet done in RCU-walk mode, the
+code is ready to release the reference when that does happen.
+
+This need to drop the reference to a symlink adds significant
+complexity. It requires a reference to the inode so that the
+`i_op->put_link()` inode operation can be called. In REF-walk, that
+reference is kept implicitly through a reference to the dentry, so
+keeping the `struct path` of the symlink is easiest. For RCU-walk,
+the pointer to the inode is kept separately. To allow switching from
+RCU-walk back to REF-walk in the middle of processing nested symlinks
+we also need the seq number for the dentry so we can confirm that
+switching back was safe.
+
+Finally, when providing a reference to a symlink, the filesystem also
+provides an opaque "cookie" that must be passed to `->put_link()` so that it
+knows what to free. This might be the allocated memory area, or a
+pointer to the `struct page` in the page cache, or something else
+completely. Only the filesystem knows what it is.
+
+In order for the reference to each symlink to be dropped when the walk completes,
+whether in RCU-walk or REF-walk, the symlink stack needs to contain,
+along with the path remnants:
+
+- the `struct path` to provide a reference to the inode in REF-walk
+- the `struct inode *` to provide a reference to the inode in RCU-walk
+- the `seq` to allow the path to be safely switched from RCU-walk to REF-walk
+- the `cookie` that tells `->put_path()` what to put.
+
+This means that each entry in the symlink stack needs to hold five
+pointers and an integer instead of just one pointer (the path
+remnant). On a 64-bit system, this is about 40 bytes per entry;
+with 40 entries it adds up to 1600 bytes total, which is less than
+half a page. So it might seem like a lot, but is by no means
+excessive.
+
+Note that, in a given stack frame, the path remnant (`name`) is not
+part of the symlink that the other fields refer to. It is the remnant
+to be followed once that symlink has been fully parsed.
+
+Following the symlink
+---------------------
+
+The main loop in `link_path_walk()` iterates seamlessly over all
+components in the path and all of the non-final symlinks. As symlinks
+are processed, the `name` pointer is adjusted to point to a new
+symlink, or is restored from the stack, so that much of the loop
+doesn't need to notice. Getting this `name` variable on and off the
+stack is very straightforward; pushing and popping the references is
+a little more complex.
+
+When a symlink is found, `walk_component()` returns the value `1`
+(`0` is returned for any other sort of success, and a negative number
+is, as usual, an error indicator). This causes `get_link()` to be
+called; it then gets the link from the filesystem. Providing that
+operation is successful, the old path `name` is placed on the stack,
+and the new value is used as the `name` for a while. When the end of
+the path is found (i.e. `*name` is `'\0'`) the old `name` is restored
+off the stack and path walking continues.
+
+Pushing and popping the reference pointers (inode, cookie, etc.) is more
+complex in part because of the desire to handle tail recursion. When
+the last component of a symlink itself points to a symlink, we
+want to pop the symlink-just-completed off the stack before pushing
+the symlink-just-found to avoid leaving empty path remnants that would
+just get in the way.
+
+It is most convenient to push the new symlink references onto the
+stack in `walk_component()` immediately when the symlink is found;
+`walk_component()` is also the last piece of code that needs to look at the
+old symlink as it walks that last component. So it is quite
+convenient for `walk_component()` to release the old symlink and pop
+the references just before pushing the reference information for the
+new symlink. It is guided in this by two flags; `WALK_GET`, which
+gives it permission to follow a symlink if it finds one, and
+`WALK_PUT`, which tells it to release the current symlink after it has been
+followed. `WALK_PUT` is tested first, leading to a call to
+`put_link()`. `WALK_GET` is tested subsequently (by
+`should_follow_link()`) leading to a call to `pick_link()` which sets
+up the stack frame.
+
+### Symlinks with no final component ###
+
+A pair of special-case symlinks deserve a little further explanation.
+Both result in a new `struct path` (with mount and dentry) being set
+up in the `nameidata`, and result in `get_link()` returning `NULL`.
+
+The more obvious case is a symlink to "`/`". All symlinks starting
+with "`/`" are detected in `get_link()` which resets the `nameidata`
+to point to the effective filesystem root. If the symlink only
+contains "`/`" then there is nothing more to do, no components at all,
+so `NULL` is returned to indicate that the symlink can be released and
+the stack frame discarded.
+
+The other case involves things in `/proc` that look like symlinks but
+aren't really.
+
+> $ ls -l /proc/self/fd/1
+> lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
+
+Every open file descriptor in any process is represented in `/proc` by
+something that looks like a symlink. It is really a reference to the
+target file, not just the name of it. When you `readlink` these
+objects you get a name that might refer to the same file - unless it
+has been unlinked or mounted over. When `walk_component()` follows
+one of these, the `->follow_link()` method in "procfs" doesn't return
+a string name, but instead calls `nd_jump_link()` which updates the
+`nameidata` in place to point to that target. `->follow_link()` then
+returns `NULL`. Again there is no final component and `get_link()`
+reports this by leaving the `last_type` field of `nameidata` as
+`LAST_BIND`.
+
+Following the symlink in the final component
+--------------------------------------------
+
+All this leads to `link_path_walk()` walking down every component, and
+following all symbolic links it finds, until it reaches the final
+component. This is just returned in the `last` field of `nameidata`.
+For some callers, this is all they need; they want to create that
+`last` name if it doesn't exist or give an error if it does. Other
+callers will want to follow a symlink if one is found, and possibly
+apply special handling to the last component of that symlink, rather
+than just the last component of the original file name. These callers
+potentially need to call `link_path_walk()` again and again on
+successive symlinks until one is found that doesn't point to another
+symlink.
+
+This case is handled by the relevant caller of `link_path_walk()`, such as
+`path_lookupat()` using a loop that calls `link_path_walk()`, and then
+handles the final component. If the final component is a symlink
+that needs to be followed, then `trailing_symlink()` is called to set
+things up properly and the loop repeats, calling `link_path_walk()`
+again. This could loop as many as 40 times if the last component of
+each symlink is another symlink.
+
+The various functions that examine the final component and possibly
+report that it is a symlink are `lookup_last()`, `mountpoint_last()`
+and `do_last()`, each of which use the same convention as
+`walk_component()` of returning `1` if a symlink was found that needs
+to be followed.
+
+Of these, `do_last()` is the most interesting as it is used for
+opening a file. Part of `do_last()` runs with `i_mutex` held and this
+part is in a separate function: `lookup_open()`.
+
+Explaining `do_last()` completely is beyond the scope of this article,
+but a few highlights should help those interested in exploring the
+code.
+
+1. Rather than just finding the target file, `do_last()` needs to open
+ it. If the file was found in the dcache, then `vfs_open()` is used for
+ this. If not, then `lookup_open()` will either call `atomic_open()` (if
+ the filesystem provides it) to combine the final lookup with the open, or
+ will perform the separate `lookup_real()` and `vfs_create()` steps
+ directly. In the later case the actual "open" of this newly found or
+ created file will be performed by `vfs_open()`, just as if the name
+ were found in the dcache.
+
+2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information
+ wasn't quite current enough. Rather than restarting the lookup from
+ the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead,
+ giving the filesystem a chance to resolve small inconsistencies.
+ If that doesn't work, only then is the lookup restarted from the top.
+
+3. An open with O_CREAT **does** follow a symlink in the final component,
+ unlike other creation system calls (like `mkdir`). So the sequence:
+
+ > ln -s bar /tmp/foo
+ > echo hello > /tmp/foo
+
+ will create a file called `/tmp/bar`. This is not permitted if
+ `O_EXCL` is set but otherwise is handled for an O_CREAT open much
+ like for a non-creating open: `should_follow_link()` returns `1`, and
+ so does `do_last()` so that `trailing_symlink()` gets called and the
+ open process continues on the symlink that was found.
+
+Updating the access time
+------------------------
+
+We previously said of RCU-walk that it would "take no locks, increment
+no counts, leave no footprints." We have since seen that some
+"footprints" can be needed when handling symlinks as a counted
+reference (or even a memory allocation) may be needed. But these
+footprints are best kept to a minimum.
+
+One other place where walking down a symlink can involve leaving
+footprints in a way that doesn't affect directories is in updating access times.
+In Unix (and Linux) every filesystem object has a "last accessed
+time", or "`atime`". Passing through a directory to access a file
+within is not considered to be an access for the purposes of
+`atime`; only listing the contents of a directory can update its `atime`.
+Symlinks are different it seems. Both reading a symlink (with `readlink()`)
+and looking up a symlink on the way to some other destination can
+update the atime on that symlink.
+
+[clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
+
+It is not clear why this is the case; POSIX has little to say on the
+subject. The [clearest statement] is that, if a particular implementation
+updates a timestamp in a place not specified by POSIX, this must be
+documented "except that any changes caused by pathname resolution need
+not be documented". This seems to imply that POSIX doesn't really
+care about access-time updates during pathname lookup.
+
+[Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
+
+An examination of history shows that prior to [Linux 1.3.87], the ext2
+filesystem, at least, didn't update atime when following a link.
+Unfortunately we have no record of why that behavior was changed.
+
+In any case, access time must now be updated and that operation can be
+quite complex. Trying to stay in RCU-walk while doing it is best
+avoided. Fortunately it is often permitted to skip the `atime`
+update. Because `atime` updates cause performance problems in various
+areas, Linux supports the `relatime` mount option, which generally
+limits the updates of `atime` to once per day on files that aren't
+being changed (and symlinks never change once created). Even without
+`relatime`, many filesystems record `atime` with a one-second
+granularity, so only one update per second is required.
+
+It is easy to test if an `atime` update is needed while in RCU-walk
+mode and, if it isn't, the update can be skipped and RCU-walk mode
+continues. Only when an `atime` update is actually required does the
+path walk drop down to REF-walk. All of this is handled in the
+`get_link()` function.
+
+A few flags
+-----------
+
+A suitable way to wrap up this tour of pathname walking is to list
+the various flags that can be stored in the `nameidata` to guide the
+lookup process. Many of these are only meaningful on the final
+component, others reflect the current state of the pathname lookup.
+And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with
+the others. If this is not set, an empty pathname causes an error
+very early on. If it is set, empty pathnames are not considered to be
+an error.
+
+### Global state flags ###
+
+We have already met two global state flags: `LOOKUP_RCU` and
+`LOOKUP_REVAL`. These select between one of three overall approaches
+to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
+
+`LOOKUP_PARENT` indicates that the final component hasn't been reached
+yet. This is primarily used to tell the audit subsystem the full
+context of a particular access being audited.
+
+`LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was
+provided by the caller, so it shouldn't be released when it is no
+longer needed.
+
+`LOOKUP_JUMPED` means that the current dentry was chosen not because
+it had the right name but for some other reason. This happens when
+following "`..`", following a symlink to `/`, crossing a mount point
+or accessing a "`/proc/$PID/fd/$FD`" symlink. In this case the
+filesystem has not been asked to revalidate the name (with
+`d_revalidate()`). In such cases the inode may still need to be
+revalidated, so `d_op->d_weak_revalidate()` is called if
+`LOOKUP_JUMPED` is set when the look completes - which may be at the
+final component or, when creating, unlinking, or renaming, at the penultimate component.
+
+### Final-component flags ###
+
+Some of these flags are only set when the final component is being
+considered. Others are only checked for when considering that final
+component.
+
+`LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount
+point, then the mount is triggered. Some operations would trigger it
+anyway, but operations like `stat()` deliberately don't. `statfs()`
+needs to trigger the mount but otherwise behaves a lot like `stat()`, so
+it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of
+"`mount --bind`".
+
+`LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for
+symlinks. Some system calls set or clear it implicitly, while
+others have API flags such as `AT_SYMLINK_FOLLOW` and
+`UMOUNT_NOFOLLOW` to control it. Its effect is similar to
+`WALK_GET` that we already met, but it is used in a different way.
+
+`LOOKUP_DIRECTORY` insists that the final component is a directory.
+Various callers set this and it is also set when the final component
+is found to be followed by a slash.
+
+Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and
+`LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made
+available to the filesystem and particularly the `->d_revalidate()`
+method. A filesystem can choose not to bother revalidating too hard
+if it knows that it will be asked to open or create the file soon.
+These flags were previously useful for `->lookup()` too but with the
+introduction of `->atomic_open()` they are less relevant there.
+
+End of the road
+---------------
+
+Despite its complexity, all this pathname lookup code appears to be
+in good shape - various parts are certainly easier to understand now
+than even a couple of releases ago. But that doesn't mean it is
+"finished". As already mentioned, RCU-walk currently only follows
+symlinks that are stored in the inode so, while it handles many ext4
+symlinks, it doesn't help with NFS, XFS, or Btrfs. That support
+is not likely to be long delayed.
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