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+===============
+Pathname lookup
+===============
+
+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.
+It has subsequently been updated to reflect changes in the kernel
+including:
+
+- per-directory parallel name lookup.
+- ``openat2()`` resolution restriction flags.
+
+Introduction to pathname lookup
+===============================
+
+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
+"``*at()``" system calls such as `openat() <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 "``*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() <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: https://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.
+
+``rename_lock`` is also used to detect and defend against potential attacks
+against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
+the parent directory is moved outside the root, bypassing the ``path_equal()``
+check). If ``rename_lock`` is updated during the lookup and the path encounters
+a "..", a potential attack occurred and ``handle_dots()`` will bail out with
+``-EAGAIN``.
+
+inode->i_rwsem
+~~~~~~~~~~~~~~
+
+``i_rwsem`` is a read/write semaphore 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 or, optionally, when the list of entries in a
+directory is being retrieved with ``readdir()``.
+
+This has a complementary role to that of ``d_lock``: ``i_rwsem`` 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_rwsem`` 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_rwsem`` plays no role.
+
+The semaphore affects pathname lookup in two distinct ways.  Firstly it
+prevents changes during 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 a shared lock on ``i_rwsem``, 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 an exclusive lock on ``i_rwsem`` before performing the last lookup so
+that the required exclusion can be achieved.  How path lookup chooses
+to take, or not take, ``i_rwsem`` is one of the
+issues addressed in a subsequent section.
+
+If two threads attempt to look up the same name at the same time - a
+name that is not yet in the dcache - the shared lock on ``i_rwsem`` will
+not prevent them both adding new dentries with the same name.  As this
+would result in confusion an extra level of interlocking is used,
+based around a secondary hash table (``in_lookup_hashtable``) and a
+per-dentry flag bit (``DCACHE_PAR_LOOKUP``).
+
+To add a new dentry to the cache while only holding a shared lock on
+``i_rwsem``, a thread must call ``d_alloc_parallel()``.  This allocates a
+dentry, stores the required name and parent in it, checks if there
+is already a matching dentry in the primary or secondary hash
+tables, and if not, stores the newly allocated dentry in the secondary
+hash table, with ``DCACHE_PAR_LOOKUP`` set.
+
+If a matching dentry was found in the primary hash table then that is
+returned and the caller can know that it lost a race with some other
+thread adding the entry.  If no matching dentry is found in either
+cache, the newly allocated dentry is returned and the caller can
+detect this from the presence of ``DCACHE_PAR_LOOKUP``.  In this case it
+knows that it has won any race and now is responsible for asking the
+filesystem to perform the lookup and find the matching inode.  When
+the lookup is complete, it must call ``d_lookup_done()`` which clears
+the flag and does some other house keeping, including removing the
+dentry from the secondary hash table - it will normally have been
+added to the primary hash table already.  Note that a ``struct
+waitqueue_head`` is passed to ``d_alloc_parallel()``, and
+``d_lookup_done()`` must be called while this ``waitqueue_head`` is still
+in scope.
+
+If a matching dentry is found in the secondary hash table,
+``d_alloc_parallel()`` has a little more work to do. It first waits for
+``DCACHE_PAR_LOOKUP`` to be cleared, using a wait_queue that was passed
+to the instance of ``d_alloc_parallel()`` that won the race and that
+will be woken by the call to ``d_lookup_done()``.  It then checks to see
+if the dentry has now been added to the primary hash table.  If it
+has, the dentry is returned and the caller just sees that it lost any
+race.  If it hasn't been added to the primary hash table, the most
+likely explanation is that some other dentry was added instead using
+``d_splice_alias()``.  In any case, ``d_alloc_parallel()`` repeats all the
+look ups from the start and will normally return something from the
+primary hash table.
+
+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.
+
+``mount_lock`` is also used to detect and defend against potential attacks
+against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
+the parent directory is moved outside the root, bypassing the ``path_equal()``
+check). If ``mount_lock`` is updated during the lookup and the path encounters
+a "..", a potential attack occurred and ``handle_dots()`` will bail out with
+``-EAGAIN``.
+
+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: https://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`` or ``LAST_DOTDOT``.
+The ``last`` field is only valid if the type is ``LAST_NORM``.
+
+``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.
+
+It should be noted that in the case of ``LOOKUP_IN_ROOT`` or
+``LOOKUP_BENEATH``, the effective root becomes the directory file descriptor
+passed to ``openat2()`` (which exposes these ``LOOKUP_`` flags).
+
+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 ``i_rwsem``, 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_rwsem`` 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_rwsem``, 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 autofs.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_rwsem`` or even ``rename_lock``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A semaphore is a fairly heavyweight lock that can only be taken when it is
+permissible to sleep.  As ``rcu_read_lock()`` forbids sleeping,
+``inode->i_rwsem`` plays no role in RCU-walk.  If some other thread does
+take ``i_rwsem`` 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  `READ_ONCE() <READ_ONCE_>`_ to access fields, and verifying the
+result is not NULL before using it.  This pattern can be seen 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 (and are therefore commonly referred to as "magic-links")::
+
+     $ 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_rwsem`` 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: https://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 some
+apply restrictions to all path components encountered in the path 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 (also known as a "magic
+link"). 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.
+
+Resolution-restriction flags
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to allow userspace to protect itself against certain race conditions
+and attack scenarios involving changing path components, a series of flags are
+available which apply restrictions to all path components encountered during
+path lookup. These flags are exposed through ``openat2()``'s ``resolve`` field.
+
+``LOOKUP_NO_SYMLINKS`` blocks all symlink traversals (including magic-links).
+This is distinctly different from ``LOOKUP_FOLLOW``, because the latter only
+relates to restricting the following of trailing symlinks.
+
+``LOOKUP_NO_MAGICLINKS`` blocks all magic-link traversals. Filesystems must
+ensure that they return errors from ``nd_jump_link()``, because that is how
+``LOOKUP_NO_MAGICLINKS`` and other magic-link restrictions are implemented.
+
+``LOOKUP_NO_XDEV`` blocks all ``vfsmount`` traversals (this includes both
+bind-mounts and ordinary mounts). Note that the ``vfsmount`` which contains the
+lookup is determined by the first mountpoint the path lookup reaches --
+absolute paths start with the ``vfsmount`` of ``/``, and relative paths start
+with the ``dfd``'s ``vfsmount``. Magic-links are only permitted if the
+``vfsmount`` of the path is unchanged.
+
+``LOOKUP_BENEATH`` blocks any path components which resolve outside the
+starting point of the resolution. This is done by blocking ``nd_jump_root()``
+as well as blocking ".." if it would jump outside the starting point.
+``rename_lock`` and ``mount_lock`` are used to detect attacks against the
+resolution of "..". Magic-links are also blocked.
+
+``LOOKUP_IN_ROOT`` resolves all path components as though the starting point
+were the filesystem root. ``nd_jump_root()`` brings the resolution back to
+the starting point, and ".." at the starting point will act as a no-op. As with
+``LOOKUP_BENEATH``, ``rename_lock`` and ``mount_lock`` are used to detect
+attacks against ".." resolution. Magic-links are also blocked.
+
+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.