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4	
5	Pathname lookup in Linux.
6	=========================
7	
8	This write-up is based on three articles published at lwn.net:
9	
10	- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
11	- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
12	- <https://lwn.net/Articles/650786/> A walk among the symlinks
13	
14	Written by Neil Brown with help from Al Viro and Jon Corbet.
15	
16	Introduction
17	------------
18	
19	The most obvious aspect of pathname lookup, which very little
20	exploration is needed to discover, is that it is complex.  There are
21	many rules, special cases, and implementation alternatives that all
22	combine to confuse the unwary reader.  Computer science has long been
23	acquainted with such complexity and has tools to help manage it.  One
24	tool that we will make extensive use of is "divide and conquer".  For
25	the early parts of the analysis we will divide off symlinks - leaving
26	them until the final part.  Well before we get to symlinks we have
27	another major division based on the VFS's approach to locking which
28	will allow us to review "REF-walk" and "RCU-walk" separately.  But we
29	are getting ahead of ourselves.  There are some important low level
30	distinctions we need to clarify first.
31	
32	There are two sorts of ...
33	--------------------------
34	
35	[`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html
36	
37	Pathnames (sometimes "file names"), used to identify objects in the
38	filesystem, will be familiar to most readers.  They contain two sorts
39	of elements: "slashes" that are sequences of one or more "`/`"
40	characters, and "components" that are sequences of one or more
41	non-"`/`" characters.  These form two kinds of paths.  Those that
42	start with slashes are "absolute" and start from the filesystem root.
43	The others are "relative" and start from the current directory, or
44	from some other location specified by a file descriptor given to a
45	"xxx`at`" system call such as "[`openat()`]".
46	
47	[`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html
48	
49	It is tempting to describe the second kind as starting with a
50	component, but that isn't always accurate: a pathname can lack both
51	slashes and components, it can be empty, in other words.  This is
52	generally forbidden in POSIX, but some of those "xxx`at`" system calls
53	in Linux permit it when the `AT_EMPTY_PATH` flag is given.  For
54	example, if you have an open file descriptor on an executable file you
55	can execute it by calling [`execveat()`] passing the file descriptor,
56	an empty path, and the `AT_EMPTY_PATH` flag.
57	
58	These paths can be divided into two sections: the final component and
59	everything else.  The "everything else" is the easy bit.  In all cases
60	it must identify a directory that already exists, otherwise an error
61	such as `ENOENT` or `ENOTDIR` will be reported.
62	
63	The final component is not so simple.  Not only do different system
64	calls interpret it quite differently (e.g. some create it, some do
65	not), but it might not even exist: neither the empty pathname nor the
66	pathname that is just slashes have a final component.  If it does
67	exist, it could be "`.`" or "`..`" which are handled quite differently
68	from other components.
69	
70	[POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
71	
72	If a pathname ends with a slash, such as "`/tmp/foo/`" it might be
73	tempting to consider that to have an empty final component.  In many
74	ways that would lead to correct results, but not always.  In
75	particular, `mkdir()` and `rmdir()` each create or remove a directory named
76	by the final component, and they are required to work with pathnames
77	ending in "`/`".  According to [POSIX]
78	
79	> A pathname that contains at least one non- &lt;slash> character and
80	> that ends with one or more trailing &lt;slash> characters shall not
81	> be resolved successfully unless the last pathname component before
82	> the trailing <slash> characters names an existing directory or a
83	> directory entry that is to be created for a directory immediately
84	> after the pathname is resolved.
85	
86	The Linux pathname walking code (mostly in `fs/namei.c`) deals with
87	all of these issues: breaking the path into components, handling the
88	"everything else" quite separately from the final component, and
89	checking that the trailing slash is not used where it isn't
90	permitted.  It also addresses the important issue of concurrent
91	access.
92	
93	While one process is looking up a pathname, another might be making
94	changes that affect that lookup.  One fairly extreme case is that if
95	"a/b" were renamed to "a/c/b" while another process were looking up
96	"a/b/..", that process might successfully resolve on "a/c".
97	Most races are much more subtle, and a big part of the task of
98	pathname lookup is to prevent them from having damaging effects.  Many
99	of the possible races are seen most clearly in the context of the
100	"dcache" and an understanding of that is central to understanding
101	pathname lookup.
102	
103	More than just a cache.
104	-----------------------
105	
106	The "dcache" caches information about names in each filesystem to
107	make them quickly available for lookup.  Each entry (known as a
108	"dentry") contains three significant fields: a component name, a
109	pointer to a parent dentry, and a pointer to the "inode" which
110	contains further information about the object in that parent with
111	the given name.  The inode pointer can be `NULL` indicating that the
112	name doesn't exist in the parent.  While there can be linkage in the
113	dentry of a directory to the dentries of the children, that linkage is
114	not used for pathname lookup, and so will not be considered here.
115	
116	The dcache has a number of uses apart from accelerating lookup.  One
117	that will be particularly relevant is that it is closely integrated
118	with the mount table that records which filesystem is mounted where.
119	What the mount table actually stores is which dentry is mounted on top
120	of which other dentry.
121	
122	When considering the dcache, we have another of our "two types"
123	distinctions: there are two types of filesystems.
124	
125	Some filesystems ensure that the information in the dcache is always
126	completely accurate (though not necessarily complete).  This can allow
127	the VFS to determine if a particular file does or doesn't exist
128	without checking with the filesystem, and means that the VFS can
129	protect the filesystem against certain races and other problems.
130	These are typically "local" filesystems such as ext3, XFS, and Btrfs.
131	
132	Other filesystems don't provide that guarantee because they cannot.
133	These are typically filesystems that are shared across a network,
134	whether remote filesystems like NFS and 9P, or cluster filesystems
135	like ocfs2 or cephfs.  These filesystems allow the VFS to revalidate
136	cached information, and must provide their own protection against
137	awkward races.  The VFS can detect these filesystems by the
138	`DCACHE_OP_REVALIDATE` flag being set in the dentry.
139	
140	REF-walk: simple concurrency management with refcounts and spinlocks
141	--------------------------------------------------------------------
142	
143	With all of those divisions carefully classified, we can now start
144	looking at the actual process of walking along a path.  In particular
145	we will start with the handling of the "everything else" part of a
146	pathname, and focus on the "REF-walk" approach to concurrency
147	management.  This code is found in the `link_path_walk()` function, if
148	you ignore all the places that only run when "`LOOKUP_RCU`"
149	(indicating the use of RCU-walk) is set.
150	
151	[Meet the Lockers]: https://lwn.net/Articles/453685/
152	
153	REF-walk is fairly heavy-handed with locks and reference counts.  Not
154	as heavy-handed as in the old "big kernel lock" days, but certainly not
155	afraid of taking a lock when one is needed.  It uses a variety of
156	different concurrency controls.  A background understanding of the
157	various primitives is assumed, or can be gleaned from elsewhere such
158	as in [Meet the Lockers].
159	
160	The locking mechanisms used by REF-walk include:
161	
162	### dentry->d_lockref ###
163	
164	This uses the lockref primitive to provide both a spinlock and a
165	reference count.  The special-sauce of this primitive is that the
166	conceptual sequence "lock; inc_ref; unlock;" can often be performed
167	with a single atomic memory operation.
168	
169	Holding a reference on a dentry ensures that the dentry won't suddenly
170	be freed and used for something else, so the values in various fields
171	will behave as expected.  It also protects the `->d_inode` reference
172	to the inode to some extent.
173	
174	The association between a dentry and its inode is fairly permanent.
175	For example, when a file is renamed, the dentry and inode move
176	together to the new location.  When a file is created the dentry will
177	initially be negative (i.e. `d_inode` is `NULL`), and will be assigned
178	to the new inode as part of the act of creation.
179	
180	When a file is deleted, this can be reflected in the cache either by
181	setting `d_inode` to `NULL`, or by removing it from the hash table
182	(described shortly) used to look up the name in the parent directory.
183	If the dentry is still in use the second option is used as it is
184	perfectly legal to keep using an open file after it has been deleted
185	and having the dentry around helps.  If the dentry is not otherwise in
186	use (i.e. if the refcount in `d_lockref` is one), only then will
187	`d_inode` be set to `NULL`.  Doing it this way is more efficient for a
188	very common case.
189	
190	So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode`
191	value will never be changed.
192	
193	### dentry->d_lock ###
194	
195	`d_lock` is a synonym for the spinlock that is part of `d_lockref` above.
196	For our purposes, holding this lock protects against the dentry being
197	renamed or unlinked.  In particular, its parent (`d_parent`), and its
198	name (`d_name`) cannot be changed, and it cannot be removed from the
199	dentry hash table.
200	
201	When looking for a name in a directory, REF-walk takes `d_lock` on
202	each candidate dentry that it finds in the hash table and then checks
203	that the parent and name are correct.  So it doesn't lock the parent
204	while searching in the cache; it only locks children.
205	
206	When looking for the parent for a given name (to handle "`..`"),
207	REF-walk can take `d_lock` to get a stable reference to `d_parent`,
208	but it first tries a more lightweight approach.  As seen in
209	`dget_parent()`, if a reference can be claimed on the parent, and if
210	subsequently `d_parent` can be seen to have not changed, then there is
211	no need to actually take the lock on the child.
212	
213	### rename_lock ###
214	
215	Looking up a given name in a given directory involves computing a hash
216	from the two values (the name and the dentry of the directory),
217	accessing that slot in a hash table, and searching the linked list
218	that is found there.
219	
220	When a dentry is renamed, the name and the parent dentry can both
221	change so the hash will almost certainly change too.  This would move the
222	dentry to a different chain in the hash table.  If a filename search
223	happened to be looking at a dentry that was moved in this way,
224	it might end up continuing the search down the wrong chain,
225	and so miss out on part of the correct chain.
226	
227	The name-lookup process (`d_lookup()`) does _not_ try to prevent this
228	from happening, but only to detect when it happens.
229	`rename_lock` is a seqlock that is updated whenever any dentry is
230	renamed.  If `d_lookup` finds that a rename happened while it
231	unsuccessfully scanned a chain in the hash table, it simply tries
232	again.
233	
234	### inode->i_mutex ###
235	
236	`i_mutex` is a mutex that serializes all changes to a particular
237	directory.  This ensures that, for example, an `unlink()` and a `rename()`
238	cannot both happen at the same time.  It also keeps the directory
239	stable while the filesystem is asked to look up a name that is not
240	currently in the dcache.
241	
242	This has a complementary role to that of `d_lock`: `i_mutex` on a
243	directory protects all of the names in that directory, while `d_lock`
244	on a name protects just one name in a directory.  Most changes to the
245	dcache hold `i_mutex` on the relevant directory inode and briefly take
246	`d_lock` on one or more the dentries while the change happens.  One
247	exception is when idle dentries are removed from the dcache due to
248	memory pressure.  This uses `d_lock`, but `i_mutex` plays no role.
249	
250	The mutex affects pathname lookup in two distinct ways.  Firstly it
251	serializes lookup of a name in a directory.  `walk_component()` uses
252	`lookup_fast()` first which, in turn, checks to see if the name is in the cache,
253	using only `d_lock` locking.  If the name isn't found, then `walk_component()`
254	falls back to `lookup_slow()` which takes `i_mutex`, checks again that
255	the name isn't in the cache, and then calls in to the filesystem to get a
256	definitive answer.  A new dentry will be added to the cache regardless of
257	the result.
258	
259	Secondly, when pathname lookup reaches the final component, it will
260	sometimes need to take `i_mutex` before performing the last lookup so
261	that the required exclusion can be achieved.  How path lookup chooses
262	to take, or not take, `i_mutex` is one of the
263	issues addressed in a subsequent section.
264	
265	### mnt->mnt_count ###
266	
267	`mnt_count` is a per-CPU reference counter on "`mount`" structures.
268	Per-CPU here means that incrementing the count is cheap as it only
269	uses CPU-local memory, but checking if the count is zero is expensive as
270	it needs to check with every CPU.  Taking a `mnt_count` reference
271	prevents the mount structure from disappearing as the result of regular
272	unmount operations, but does not prevent a "lazy" unmount.  So holding
273	`mnt_count` doesn't ensure that the mount remains in the namespace and,
274	in particular, doesn't stabilize the link to the mounted-on dentry.  It
275	does, however, ensure that the `mount` data structure remains coherent,
276	and it provides a reference to the root dentry of the mounted
277	filesystem.  So a reference through `->mnt_count` provides a stable
278	reference to the mounted dentry, but not the mounted-on dentry.
279	
280	### mount_lock ###
281	
282	`mount_lock` is a global seqlock, a bit like `rename_lock`.  It can be used to
283	check if any change has been made to any mount points.
284	
285	While walking down the tree (away from the root) this lock is used when
286	crossing a mount point to check that the crossing was safe.  That is,
287	the value in the seqlock is read, then the code finds the mount that
288	is mounted on the current directory, if there is one, and increments
289	the `mnt_count`.  Finally the value in `mount_lock` is checked against
290	the old value.  If there is no change, then the crossing was safe.  If there
291	was a change, the `mnt_count` is decremented and the whole process is
292	retried.
293	
294	When walking up the tree (towards the root) by following a ".." link,
295	a little more care is needed.  In this case the seqlock (which
296	contains both a counter and a spinlock) is fully locked to prevent
297	any changes to any mount points while stepping up.  This locking is
298	needed to stabilize the link to the mounted-on dentry, which the
299	refcount on the mount itself doesn't ensure.
300	
301	### RCU ###
302	
303	Finally the global (but extremely lightweight) RCU read lock is held
304	from time to time to ensure certain data structures don't get freed
305	unexpectedly.
306	
307	In particular it is held while scanning chains in the dcache hash
308	table, and the mount point hash table.
309	
310	Bringing it together with `struct nameidata`
311	--------------------------------------------
312	
313	[First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
314	
315	Throughout the process of walking a path, the current status is stored
316	in a `struct nameidata`, "namei" being the traditional name - dating
317	all the way back to [First Edition Unix] - of the function that
318	converts a "name" to an "inode".  `struct nameidata` contains (among
319	other fields):
320	
321	### `struct path path` ###
322	
323	A `path` contains a `struct vfsmount` (which is
324	embedded in a `struct mount`) and a `struct dentry`.  Together these
325	record the current status of the walk.  They start out referring to the
326	starting point (the current working directory, the root directory, or some other
327	directory identified by a file descriptor), and are updated on each
328	step.  A reference through `d_lockref` and `mnt_count` is always
329	held.
330	
331	### `struct qstr last` ###
332	
333	This is a string together with a length (i.e. _not_ `nul` terminated)
334	that is the "next" component in the pathname.
335	
336	### `int last_type` ###
337	
338	This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or
339	`LAST_BIND`.  The `last` field is only valid if the type is
340	`LAST_NORM`.  `LAST_BIND` is used when following a symlink and no
341	components of the symlink have been processed yet.  Others should be
342	fairly self-explanatory.
343	
344	### `struct path root` ###
345	
346	This is used to hold a reference to the effective root of the
347	filesystem.  Often that reference won't be needed, so this field is
348	only assigned the first time it is used, or when a non-standard root
349	is requested.  Keeping a reference in the `nameidata` ensures that
350	only one root is in effect for the entire path walk, even if it races
351	with a `chroot()` system call.
352	
353	The root is needed when either of two conditions holds: (1) either the
354	pathname or a symbolic link starts with a "'/'", or (2) a "`..`"
355	component is being handled, since "`..`" from the root must always stay
356	at the root.  The value used is usually the current root directory of
357	the calling process.  An alternate root can be provided as when
358	`sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call
359	`mount_subtree()`.  In each case a pathname is being looked up in a very
360	specific part of the filesystem, and the lookup must not be allowed to
361	escape that subtree.  It works a bit like a local `chroot()`.
362	
363	Ignoring the handling of symbolic links, we can now describe the
364	"`link_path_walk()`" function, which handles the lookup of everything
365	except the final component as:
366	
367	>  Given a path (`name`) and a nameidata structure (`nd`), check that the
368	>  current directory has execute permission and then advance `name`
369	>  over one component while updating `last_type` and `last`.  If that
370	>  was the final component, then return, otherwise call
371	>  `walk_component()` and repeat from the top.
372	
373	`walk_component()` is even easier.  If the component is `LAST_DOTS`,
374	it calls `handle_dots()` which does the necessary locking as already
375	described.  If it finds a `LAST_NORM` component it first calls
376	"`lookup_fast()`" which only looks in the dcache, but will ask the
377	filesystem to revalidate the result if it is that sort of filesystem.
378	If that doesn't get a good result, it calls "`lookup_slow()`" which
379	takes the `i_mutex`, rechecks the cache, and then asks the filesystem
380	to find a definitive answer.  Each of these will call
381	`follow_managed()` (as described below) to handle any mount points.
382	
383	In the absence of symbolic links, `walk_component()` creates a new
384	`struct path` containing a counted reference to the new dentry and a
385	reference to the new `vfsmount` which is only counted if it is
386	different from the previous `vfsmount`.  It then calls
387	`path_to_nameidata()` to install the new `struct path` in the
388	`struct nameidata` and drop the unneeded references.
389	
390	This "hand-over-hand" sequencing of getting a reference to the new
391	dentry before dropping the reference to the previous dentry may
392	seem obvious, but is worth pointing out so that we will recognize its
393	analogue in the "RCU-walk" version.
394	
395	Handling the final component.
396	-----------------------------
397	
398	`link_path_walk()` only walks as far as setting `nd->last` and
399	`nd->last_type` to refer to the final component of the path.  It does
400	not call `walk_component()` that last time.  Handling that final
401	component remains for the caller to sort out. Those callers are
402	`path_lookupat()`, `path_parentat()`, `path_mountpoint()` and
403	`path_openat()` each of which handles the differing requirements of
404	different system calls.
405	
406	`path_parentat()` is clearly the simplest - it just wraps a little bit
407	of housekeeping around `link_path_walk()` and returns the parent
408	directory and final component to the caller.  The caller will be either
409	aiming to create a name (via `filename_create()`) or remove or rename
410	a name (in which case `user_path_parent()` is used).  They will use
411	`i_mutex` to exclude other changes while they validate and then
412	perform their operation.
413	
414	`path_lookupat()` is nearly as simple - it is used when an existing
415	object is wanted such as by `stat()` or `chmod()`.  It essentially just
416	calls `walk_component()` on the final component through a call to
417	`lookup_last()`.  `path_lookupat()` returns just the final dentry.
418	
419	`path_mountpoint()` handles the special case of unmounting which must
420	not try to revalidate the mounted filesystem.  It effectively
421	contains, through a call to `mountpoint_last()`, an alternate
422	implementation of `lookup_slow()` which skips that step.  This is
423	important when unmounting a filesystem that is inaccessible, such as
424	one provided by a dead NFS server.
425	
426	Finally `path_openat()` is used for the `open()` system call; it
427	contains, in support functions starting with "`do_last()`", all the
428	complexity needed to handle the different subtleties of O_CREAT (with
429	or without O_EXCL), final "`/`" characters, and trailing symbolic
430	links.  We will revisit this in the final part of this series, which
431	focuses on those symbolic links.  "`do_last()`" will sometimes, but
432	not always, take `i_mutex`, depending on what it finds.
433	
434	Each of these, or the functions which call them, need to be alert to
435	the possibility that the final component is not `LAST_NORM`.  If the
436	goal of the lookup is to create something, then any value for
437	`last_type` other than `LAST_NORM` will result in an error.  For
438	example if `path_parentat()` reports `LAST_DOTDOT`, then the caller
439	won't try to create that name.  They also check for trailing slashes
440	by testing `last.name[last.len]`.  If there is any character beyond
441	the final component, it must be a trailing slash.
442	
443	Revalidation and automounts
444	---------------------------
445	
446	Apart from symbolic links, there are only two parts of the "REF-walk"
447	process not yet covered.  One is the handling of stale cache entries
448	and the other is automounts.
449	
450	On filesystems that require it, the lookup routines will call the
451	`->d_revalidate()` dentry method to ensure that the cached information
452	is current.  This will often confirm validity or update a few details
453	from a server.  In some cases it may find that there has been change
454	further up the path and that something that was thought to be valid
455	previously isn't really.  When this happens the lookup of the whole
456	path is aborted and retried with the "`LOOKUP_REVAL`" flag set.  This
457	forces revalidation to be more thorough.  We will see more details of
458	this retry process in the next article.
459	
460	Automount points are locations in the filesystem where an attempt to
461	lookup a name can trigger changes to how that lookup should be
462	handled, in particular by mounting a filesystem there.  These are
463	covered in greater detail in autofs4.txt in the Linux documentation
464	tree, but a few notes specifically related to path lookup are in order
465	here.
466	
467	The Linux VFS has a concept of "managed" dentries which is reflected
468	in function names such as "`follow_managed()`".  There are three
469	potentially interesting things about these dentries corresponding
470	to three different flags that might be set in `dentry->d_flags`:
471	
472	### `DCACHE_MANAGE_TRANSIT` ###
473	
474	If this flag has been set, then the filesystem has requested that the
475	`d_manage()` dentry operation be called before handling any possible
476	mount point.  This can perform two particular services:
477	
478	It can block to avoid races.  If an automount point is being
479	unmounted, the `d_manage()` function will usually wait for that
480	process to complete before letting the new lookup proceed and possibly
481	trigger a new automount.
482	
483	It can selectively allow only some processes to transit through a
484	mount point.  When a server process is managing automounts, it may
485	need to access a directory without triggering normal automount
486	processing.  That server process can identify itself to the `autofs`
487	filesystem, which will then give it a special pass through
488	`d_manage()` by returning `-EISDIR`.
489	
490	### `DCACHE_MOUNTED` ###
491	
492	This flag is set on every dentry that is mounted on.  As Linux
493	supports multiple filesystem namespaces, it is possible that the
494	dentry may not be mounted on in *this* namespace, just in some
495	other.  So this flag is seen as a hint, not a promise.
496	
497	If this flag is set, and `d_manage()` didn't return `-EISDIR`,
498	`lookup_mnt()` is called to examine the mount hash table (honoring the
499	`mount_lock` described earlier) and possibly return a new `vfsmount`
500	and a new `dentry` (both with counted references).
501	
502	### `DCACHE_NEED_AUTOMOUNT` ###
503	
504	If `d_manage()` allowed us to get this far, and `lookup_mnt()` didn't
505	find a mount point, then this flag causes the `d_automount()` dentry
506	operation to be called.
507	
508	The `d_automount()` operation can be arbitrarily complex and may
509	communicate with server processes etc. but it should ultimately either
510	report that there was an error, that there was nothing to mount, or
511	should provide an updated `struct path` with new `dentry` and `vfsmount`.
512	
513	In the latter case, `finish_automount()` will be called to safely
514	install the new mount point into the mount table.
515	
516	There is no new locking of import here and it is important that no
517	locks (only counted references) are held over this processing due to
518	the very real possibility of extended delays.
519	This will become more important next time when we examine RCU-walk
520	which is particularly sensitive to delays.
521	
522	RCU-walk - faster pathname lookup in Linux
523	==========================================
524	
525	RCU-walk is another algorithm for performing pathname lookup in Linux.
526	It is in many ways similar to REF-walk and the two share quite a bit
527	of code.  The significant difference in RCU-walk is how it allows for
528	the possibility of concurrent access.
529	
530	We noted that REF-walk is complex because there are numerous details
531	and special cases.  RCU-walk reduces this complexity by simply
532	refusing to handle a number of cases -- it instead falls back to
533	REF-walk.  The difficulty with RCU-walk comes from a different
534	direction: unfamiliarity.  The locking rules when depending on RCU are
535	quite different from traditional locking, so we will spend a little extra
536	time when we come to those.
537	
538	Clear demarcation of roles
539	--------------------------
540	
541	The easiest way to manage concurrency is to forcibly stop any other
542	thread from changing the data structures that a given thread is
543	looking at.  In cases where no other thread would even think of
544	changing the data and lots of different threads want to read at the
545	same time, this can be very costly.  Even when using locks that permit
546	multiple concurrent readers, the simple act of updating the count of
547	the number of current readers can impose an unwanted cost.  So the
548	goal when reading a shared data structure that no other process is
549	changing is to avoid writing anything to memory at all.  Take no
550	locks, increment no counts, leave no footprints.
551	
552	The REF-walk mechanism already described certainly doesn't follow this
553	principle, but then it is really designed to work when there may well
554	be other threads modifying the data.  RCU-walk, in contrast, is
555	designed for the common situation where there are lots of frequent
556	readers and only occasional writers.  This may not be common in all
557	parts of the filesystem tree, but in many parts it will be.  For the
558	other parts it is important that RCU-walk can quickly fall back to
559	using REF-walk.
560	
561	Pathname lookup always starts in RCU-walk mode but only remains there
562	as long as what it is looking for is in the cache and is stable.  It
563	dances lightly down the cached filesystem image, leaving no footprints
564	and carefully watching where it is, to be sure it doesn't trip.  If it
565	notices that something has changed or is changing, or if something
566	isn't in the cache, then it tries to stop gracefully and switch to
567	REF-walk.
568	
569	This stopping requires getting a counted reference on the current
570	`vfsmount` and `dentry`, and ensuring that these are still valid -
571	that a path walk with REF-walk would have found the same entries.
572	This is an invariant that RCU-walk must guarantee.  It can only make
573	decisions, such as selecting the next step, that are decisions which
574	REF-walk could also have made if it were walking down the tree at the
575	same time.  If the graceful stop succeeds, the rest of the path is
576	processed with the reliable, if slightly sluggish, REF-walk.  If
577	RCU-walk finds it cannot stop gracefully, it simply gives up and
578	restarts from the top with REF-walk.
579	
580	This pattern of "try RCU-walk, if that fails try REF-walk" can be
581	clearly seen in functions like `filename_lookup()`,
582	`filename_parentat()`, `filename_mountpoint()`,
583	`do_filp_open()`, and `do_file_open_root()`.  These five
584	correspond roughly to the four `path_`* functions we met earlier,
585	each of which calls `link_path_walk()`.  The `path_*` functions are
586	called using different mode flags until a mode is found which works.
587	They are first called with `LOOKUP_RCU` set to request "RCU-walk".  If
588	that fails with the error `ECHILD` they are called again with no
589	special flag to request "REF-walk".  If either of those report the
590	error `ESTALE` a final attempt is made with `LOOKUP_REVAL` set (and no
591	`LOOKUP_RCU`) to ensure that entries found in the cache are forcibly
592	revalidated - normally entries are only revalidated if the filesystem
593	determines that they are too old to trust.
594	
595	The `LOOKUP_RCU` attempt may drop that flag internally and switch to
596	REF-walk, but will never then try to switch back to RCU-walk.  Places
597	that trip up RCU-walk are much more likely to be near the leaves and
598	so it is very unlikely that there will be much, if any, benefit from
599	switching back.
600	
601	RCU and seqlocks: fast and light
602	--------------------------------
603	
604	RCU is, unsurprisingly, critical to RCU-walk mode.  The
605	`rcu_read_lock()` is held for the entire time that RCU-walk is walking
606	down a path.  The particular guarantee it provides is that the key
607	data structures - dentries, inodes, super_blocks, and mounts - will
608	not be freed while the lock is held.  They might be unlinked or
609	invalidated in one way or another, but the memory will not be
610	repurposed so values in various fields will still be meaningful.  This
611	is the only guarantee that RCU provides; everything else is done using
612	seqlocks.
613	
614	As we saw above, REF-walk holds a counted reference to the current
615	dentry and the current vfsmount, and does not release those references
616	before taking references to the "next" dentry or vfsmount.  It also
617	sometimes takes the `d_lock` spinlock.  These references and locks are
618	taken to prevent certain changes from happening.  RCU-walk must not
619	take those references or locks and so cannot prevent such changes.
620	Instead, it checks to see if a change has been made, and aborts or
621	retries if it has.
622	
623	To preserve the invariant mentioned above (that RCU-walk may only make
624	decisions that REF-walk could have made), it must make the checks at
625	or near the same places that REF-walk holds the references.  So, when
626	REF-walk increments a reference count or takes a spinlock, RCU-walk
627	samples the status of a seqlock using `read_seqcount_begin()` or a
628	similar function.  When REF-walk decrements the count or drops the
629	lock, RCU-walk checks if the sampled status is still valid using
630	`read_seqcount_retry()` or similar.
631	
632	However, there is a little bit more to seqlocks than that.  If
633	RCU-walk accesses two different fields in a seqlock-protected
634	structure, or accesses the same field twice, there is no a priori
635	guarantee of any consistency between those accesses.  When consistency
636	is needed - which it usually is - RCU-walk must take a copy and then
637	use `read_seqcount_retry()` to validate that copy.
638	
639	`read_seqcount_retry()` not only checks the sequence number, but also
640	imposes a memory barrier so that no memory-read instruction from
641	*before* the call can be delayed until *after* the call, either by the
642	CPU or by the compiler.  A simple example of this can be seen in
643	`slow_dentry_cmp()` which, for filesystems which do not use simple
644	byte-wise name equality, calls into the filesystem to compare a name
645	against a dentry.  The length and name pointer are copied into local
646	variables, then `read_seqcount_retry()` is called to confirm the two
647	are consistent, and only then is `->d_compare()` called.  When
648	standard filename comparison is used, `dentry_cmp()` is called
649	instead.  Notably it does _not_ use `read_seqcount_retry()`, but
650	instead has a large comment explaining why the consistency guarantee
651	isn't necessary.  A subsequent `read_seqcount_retry()` will be
652	sufficient to catch any problem that could occur at this point.
653	
654	With that little refresher on seqlocks out of the way we can look at
655	the bigger picture of how RCU-walk uses seqlocks.
656	
657	### `mount_lock` and `nd->m_seq` ###
658	
659	We already met the `mount_lock` seqlock when REF-walk used it to
660	ensure that crossing a mount point is performed safely.  RCU-walk uses
661	it for that too, but for quite a bit more.
662	
663	Instead of taking a counted reference to each `vfsmount` as it
664	descends the tree, RCU-walk samples the state of `mount_lock` at the
665	start of the walk and stores this initial sequence number in the
666	`struct nameidata` in the `m_seq` field.  This one lock and one
667	sequence number are used to validate all accesses to all `vfsmounts`,
668	and all mount point crossings.  As changes to the mount table are
669	relatively rare, it is reasonable to fall back on REF-walk any time
670	that any "mount" or "unmount" happens.
671	
672	`m_seq` is checked (using `read_seqretry()`) at the end of an RCU-walk
673	sequence, whether switching to REF-walk for the rest of the path or
674	when the end of the path is reached.  It is also checked when stepping
675	down over a mount point (in `__follow_mount_rcu()`) or up (in
676	`follow_dotdot_rcu()`).  If it is ever found to have changed, the
677	whole RCU-walk sequence is aborted and the path is processed again by
678	REF-walk.
679	
680	If RCU-walk finds that `mount_lock` hasn't changed then it can be sure
681	that, had REF-walk taken counted references on each vfsmount, the
682	results would have been the same.  This ensures the invariant holds,
683	at least for vfsmount structures.
684	
685	### `dentry->d_seq` and `nd->seq`. ###
686	
687	In place of taking a count or lock on `d_reflock`, RCU-walk samples
688	the per-dentry `d_seq` seqlock, and stores the sequence number in the
689	`seq` field of the nameidata structure, so `nd->seq` should always be
690	the current sequence number of `nd->dentry`.  This number needs to be
691	revalidated after copying, and before using, the name, parent, or
692	inode of the dentry.
693	
694	The handling of the name we have already looked at, and the parent is
695	only accessed in `follow_dotdot_rcu()` which fairly trivially follows
696	the required pattern, though it does so for three different cases.
697	
698	When not at a mount point, `d_parent` is followed and its `d_seq` is
699	collected.  When we are at a mount point, we instead follow the
700	`mnt->mnt_mountpoint` link to get a new dentry and collect its
701	`d_seq`.  Then, after finally finding a `d_parent` to follow, we must
702	check if we have landed on a mount point and, if so, must find that
703	mount point and follow the `mnt->mnt_root` link.  This would imply a
704	somewhat unusual, but certainly possible, circumstance where the
705	starting point of the path lookup was in part of the filesystem that
706	was mounted on, and so not visible from the root.
707	
708	The inode pointer, stored in `->d_inode`, is a little more
709	interesting.  The inode will always need to be accessed at least
710	twice, once to determine if it is NULL and once to verify access
711	permissions.  Symlink handling requires a validated inode pointer too.
712	Rather than revalidating on each access, a copy is made on the first
713	access and it is stored in the `inode` field of `nameidata` from where
714	it can be safely accessed without further validation.
715	
716	`lookup_fast()` is the only lookup routine that is used in RCU-mode,
717	`lookup_slow()` being too slow and requiring locks.  It is in
718	`lookup_fast()` that we find the important "hand over hand" tracking
719	of the current dentry.
720	
721	The current `dentry` and current `seq` number are passed to
722	`__d_lookup_rcu()` which, on success, returns a new `dentry` and a
723	new `seq` number.  `lookup_fast()` then copies the inode pointer and
724	revalidates the new `seq` number.  It then validates the old `dentry`
725	with the old `seq` number one last time and only then continues.  This
726	process of getting the `seq` number of the new dentry and then
727	checking the `seq` number of the old exactly mirrors the process of
728	getting a counted reference to the new dentry before dropping that for
729	the old dentry which we saw in REF-walk.
730	
731	### No `inode->i_mutex` or even `rename_lock` ###
732	
733	A mutex is a fairly heavyweight lock that can only be taken when it is
734	permissible to sleep.  As `rcu_read_lock()` forbids sleeping,
735	`inode->i_mutex` plays no role in RCU-walk.  If some other thread does
736	take `i_mutex` and modifies the directory in a way that RCU-walk needs
737	to notice, the result will be either that RCU-walk fails to find the
738	dentry that it is looking for, or it will find a dentry which
739	`read_seqretry()` won't validate.  In either case it will drop down to
740	REF-walk mode which can take whatever locks are needed.
741	
742	Though `rename_lock` could be used by RCU-walk as it doesn't require
743	any sleeping, RCU-walk doesn't bother.  REF-walk uses `rename_lock` to
744	protect against the possibility of hash chains in the dcache changing
745	while they are being searched.  This can result in failing to find
746	something that actually is there.  When RCU-walk fails to find
747	something in the dentry cache, whether it is really there or not, it
748	already drops down to REF-walk and tries again with appropriate
749	locking.  This neatly handles all cases, so adding extra checks on
750	rename_lock would bring no significant value.
751	
752	`unlazy walk()` and `complete_walk()`
753	-------------------------------------
754	
755	That "dropping down to REF-walk" typically involves a call to
756	`unlazy_walk()`, so named because "RCU-walk" is also sometimes
757	referred to as "lazy walk".  `unlazy_walk()` is called when
758	following the path down to the current vfsmount/dentry pair seems to
759	have proceeded successfully, but the next step is problematic.  This
760	can happen if the next name cannot be found in the dcache, if
761	permission checking or name revalidation couldn't be achieved while
762	the `rcu_read_lock()` is held (which forbids sleeping), if an
763	automount point is found, or in a couple of cases involving symlinks.
764	It is also called from `complete_walk()` when the lookup has reached
765	the final component, or the very end of the path, depending on which
766	particular flavor of lookup is used.
767	
768	Other reasons for dropping out of RCU-walk that do not trigger a call
769	to `unlazy_walk()` are when some inconsistency is found that cannot be
770	handled immediately, such as `mount_lock` or one of the `d_seq`
771	seqlocks reporting a change.  In these cases the relevant function
772	will return `-ECHILD` which will percolate up until it triggers a new
773	attempt from the top using REF-walk.
774	
775	For those cases where `unlazy_walk()` is an option, it essentially
776	takes a reference on each of the pointers that it holds (vfsmount,
777	dentry, and possibly some symbolic links) and then verifies that the
778	relevant seqlocks have not been changed.  If there have been changes,
779	it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk
780	has been a success and the lookup process continues.
781	
782	Taking a reference on those pointers is not quite as simple as just
783	incrementing a counter.  That works to take a second reference if you
784	already have one (often indirectly through another object), but it
785	isn't sufficient if you don't actually have a counted reference at
786	all.  For `dentry->d_lockref`, it is safe to increment the reference
787	counter to get a reference unless it has been explicitly marked as
788	"dead" which involves setting the counter to `-128`.
789	`lockref_get_not_dead()` achieves this.
790	
791	For `mnt->mnt_count` it is safe to take a reference as long as
792	`mount_lock` is then used to validate the reference.  If that
793	validation fails, it may *not* be safe to just drop that reference in
794	the standard way of calling `mnt_put()` - an unmount may have
795	progressed too far.  So the code in `legitimize_mnt()`, when it
796	finds that the reference it got might not be safe, checks the
797	`MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is
798	correct, or if it should just decrement the count and pretend none of
799	this ever happened.
800	
801	Taking care in filesystems
802	---------------------------
803	
804	RCU-walk depends almost entirely on cached information and often will
805	not call into the filesystem at all.  However there are two places,
806	besides the already-mentioned component-name comparison, where the
807	file system might be included in RCU-walk, and it must know to be
808	careful.
809	
810	If the filesystem has non-standard permission-checking requirements -
811	such as a networked filesystem which may need to check with the server
812	- the `i_op->permission` interface might be called during RCU-walk.
813	In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it
814	knows not to sleep, but to return `-ECHILD` if it cannot complete
815	promptly.  `i_op->permission` is given the inode pointer, not the
816	dentry, so it doesn't need to worry about further consistency checks.
817	However if it accesses any other filesystem data structures, it must
818	ensure they are safe to be accessed with only the `rcu_read_lock()`
819	held.  This typically means they must be freed using `kfree_rcu()` or
820	similar.
821	
822	[`READ_ONCE()`]: https://lwn.net/Articles/624126/
823	
824	If the filesystem may need to revalidate dcache entries, then
825	`d_op->d_revalidate` may be called in RCU-walk too.  This interface
826	*is* passed the dentry but does not have access to the `inode` or the
827	`seq` number from the `nameidata`, so it needs to be extra careful
828	when accessing fields in the dentry.  This "extra care" typically
829	involves using `ACCESS_ONCE()` or the newer [`READ_ONCE()`] to access
830	fields, and verifying the result is not NULL before using it.  This
831	pattern can be see in `nfs_lookup_revalidate()`.
832	
833	A pair of patterns
834	------------------
835	
836	In various places in the details of REF-walk and RCU-walk, and also in
837	the big picture, there are a couple of related patterns that are worth
838	being aware of.
839	
840	The first is "try quickly and check, if that fails try slowly".  We
841	can see that in the high-level approach of first trying RCU-walk and
842	then trying REF-walk, and in places where `unlazy_walk()` is used to
843	switch to REF-walk for the rest of the path.  We also saw it earlier
844	in `dget_parent()` when following a "`..`" link.  It tries a quick way
845	to get a reference, then falls back to taking locks if needed.
846	
847	The second pattern is "try quickly and check, if that fails try
848	again - repeatedly".  This is seen with the use of `rename_lock` and
849	`mount_lock` in REF-walk.  RCU-walk doesn't make use of this pattern -
850	if anything goes wrong it is much safer to just abort and try a more
851	sedate approach.
852	
853	The emphasis here is "try quickly and check".  It should probably be
854	"try quickly _and carefully,_ then check".  The fact that checking is
855	needed is a reminder that the system is dynamic and only a limited
856	number of things are safe at all.  The most likely cause of errors in
857	this whole process is assuming something is safe when in reality it
858	isn't.  Careful consideration of what exactly guarantees the safety of
859	each access is sometimes necessary.
860	
861	A walk among the symlinks
862	=========================
863	
864	There are several basic issues that we will examine to understand the
865	handling of symbolic links:  the symlink stack, together with cache
866	lifetimes, will help us understand the overall recursive handling of
867	symlinks and lead to the special care needed for the final component.
868	Then a consideration of access-time updates and summary of the various
869	flags controlling lookup will finish the story.
870	
871	The symlink stack
872	-----------------
873	
874	There are only two sorts of filesystem objects that can usefully
875	appear in a path prior to the final component: directories and symlinks.
876	Handling directories is quite straightforward: the new directory
877	simply becomes the starting point at which to interpret the next
878	component on the path.  Handling symbolic links requires a bit more
879	work.
880	
881	Conceptually, symbolic links could be handled by editing the path.  If
882	a component name refers to a symbolic link, then that component is
883	replaced by the body of the link and, if that body starts with a '/',
884	then all preceding parts of the path are discarded.  This is what the
885	"`readlink -f`" command does, though it also edits out "`.`" and
886	"`..`" components.
887	
888	Directly editing the path string is not really necessary when looking
889	up a path, and discarding early components is pointless as they aren't
890	looked at anyway.  Keeping track of all remaining components is
891	important, but they can of course be kept separately; there is no need
892	to concatenate them.  As one symlink may easily refer to another,
893	which in turn can refer to a third, we may need to keep the remaining
894	components of several paths, each to be processed when the preceding
895	ones are completed.  These path remnants are kept on a stack of
896	limited size.
897	
898	There are two reasons for placing limits on how many symlinks can
899	occur in a single path lookup.  The most obvious is to avoid loops.
900	If a symlink referred to itself either directly or through
901	intermediaries, then following the symlink can never complete
902	successfully - the error `ELOOP` must be returned.  Loops can be
903	detected without imposing limits, but limits are the simplest solution
904	and, given the second reason for restriction, quite sufficient.
905	
906	[outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
907	
908	The second reason was [outlined recently] by Linus:
909	
910	>  Because it's a latency and DoS issue too. We need to react well to
911	>  true loops, but also to "very deep" non-loops. It's not about memory
912	>  use, it's about users triggering unreasonable CPU resources.
913	
914	Linux imposes a limit on the length of any pathname: `PATH_MAX`, which
915	is 4096.  There are a number of reasons for this limit; not letting the
916	kernel spend too much time on just one path is one of them.  With
917	symbolic links you can effectively generate much longer paths so some
918	sort of limit is needed for the same reason.  Linux imposes a limit of
919	at most 40 symlinks in any one path lookup.  It previously imposed a
920	further limit of eight on the maximum depth of recursion, but that was
921	raised to 40 when a separate stack was implemented, so there is now
922	just the one limit.
923	
924	The `nameidata` structure that we met in an earlier article contains a
925	small stack that can be used to store the remaining part of up to two
926	symlinks.  In many cases this will be sufficient.  If it isn't, a
927	separate stack is allocated with room for 40 symlinks.  Pathname
928	lookup will never exceed that stack as, once the 40th symlink is
929	detected, an error is returned.
930	
931	It might seem that the name remnants are all that needs to be stored on
932	this stack, but we need a bit more.  To see that, we need to move on to
933	cache lifetimes.
934	
935	Storage and lifetime of cached symlinks
936	---------------------------------------
937	
938	Like other filesystem resources, such as inodes and directory
939	entries, symlinks are cached by Linux to avoid repeated costly access
940	to external storage.  It is particularly important for RCU-walk to be
941	able to find and temporarily hold onto these cached entries, so that
942	it doesn't need to drop down into REF-walk.
943	
944	[object-oriented design pattern]: https://lwn.net/Articles/446317/
945	
946	While each filesystem is free to make its own choice, symlinks are
947	typically stored in one of two places.  Short symlinks are often
948	stored directly in the inode.  When a filesystem allocates a `struct
949	inode` it typically allocates extra space to store private data (a
950	common [object-oriented design pattern] in the kernel).  This will
951	sometimes include space for a symlink.  The other common location is
952	in the page cache, which normally stores the content of files.  The
953	pathname in a symlink can be seen as the content of that symlink and
954	can easily be stored in the page cache just like file content.
955	
956	When neither of these is suitable, the next most likely scenario is
957	that the filesystem will allocate some temporary memory and copy or
958	construct the symlink content into that memory whenever it is needed.
959	
960	When the symlink is stored in the inode, it has the same lifetime as
961	the inode which, itself, is protected by RCU or by a counted reference
962	on the dentry.  This means that the mechanisms that pathname lookup
963	uses to access the dcache and icache (inode cache) safely are quite
964	sufficient for accessing some cached symlinks safely.  In these cases,
965	the `i_link` pointer in the inode is set to point to wherever the
966	symlink is stored and it can be accessed directly whenever needed.
967	
968	When the symlink is stored in the page cache or elsewhere, the
969	situation is not so straightforward.  A reference on a dentry or even
970	on an inode does not imply any reference on cached pages of that
971	inode, and even an `rcu_read_lock()` is not sufficient to ensure that
972	a page will not disappear.  So for these symlinks the pathname lookup
973	code needs to ask the filesystem to provide a stable reference and,
974	significantly, needs to release that reference when it is finished
975	with it.
976	
977	Taking a reference to a cache page is often possible even in RCU-walk
978	mode.  It does require making changes to memory, which is best avoided,
979	but that isn't necessarily a big cost and it is better than dropping
980	out of RCU-walk mode completely.  Even filesystems that allocate
981	space to copy the symlink into can use `GFP_ATOMIC` to often successfully
982	allocate memory without the need to drop out of RCU-walk.  If a
983	filesystem cannot successfully get a reference in RCU-walk mode, it
984	must return `-ECHILD` and `unlazy_walk()` will be called to return to
985	REF-walk mode in which the filesystem is allowed to sleep.
986	
987	The place for all this to happen is the `i_op->follow_link()` inode
988	method.  In the present mainline code this is never actually called in
989	RCU-walk mode as the rewrite is not quite complete.  It is likely that
990	in a future release this method will be passed an `inode` pointer when
991	called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
992	validated pointer.  Much like the `i_op->permission()` method we
993	looked at previously, `->follow_link()` would need to be careful that
994	all the data structures it references are safe to be accessed while
995	holding no counted reference, only the RCU lock.  Though getting a
996	reference with `->follow_link()` is not yet done in RCU-walk mode, the
997	code is ready to release the reference when that does happen.
998	
999	This need to drop the reference to a symlink adds significant
1000	complexity.  It requires a reference to the inode so that the
1001	`i_op->put_link()` inode operation can be called.  In REF-walk, that
1002	reference is kept implicitly through a reference to the dentry, so
1003	keeping the `struct path` of the symlink is easiest.  For RCU-walk,
1004	the pointer to the inode is kept separately.  To allow switching from
1005	RCU-walk back to REF-walk in the middle of processing nested symlinks
1006	we also need the seq number for the dentry so we can confirm that
1007	switching back was safe.
1008	
1009	Finally, when providing a reference to a symlink, the filesystem also
1010	provides an opaque "cookie" that must be passed to `->put_link()` so that it
1011	knows what to free.  This might be the allocated memory area, or a
1012	pointer to the `struct page` in the page cache, or something else
1013	completely.  Only the filesystem knows what it is.
1014	
1015	In order for the reference to each symlink to be dropped when the walk completes,
1016	whether in RCU-walk or REF-walk, the symlink stack needs to contain,
1017	along with the path remnants:
1018	
1019	- the `struct path` to provide a reference to the inode in REF-walk
1020	- the `struct inode *` to provide a reference to the inode in RCU-walk
1021	- the `seq` to allow the path to be safely switched from RCU-walk to REF-walk
1022	- the `cookie` that tells `->put_path()` what to put.
1023	
1024	This means that each entry in the symlink stack needs to hold five
1025	pointers and an integer instead of just one pointer (the path
1026	remnant).  On a 64-bit system, this is about 40 bytes per entry;
1027	with 40 entries it adds up to 1600 bytes total, which is less than
1028	half a page.  So it might seem like a lot, but is by no means
1029	excessive.
1030	
1031	Note that, in a given stack frame, the path remnant (`name`) is not
1032	part of the symlink that the other fields refer to.  It is the remnant
1033	to be followed once that symlink has been fully parsed.
1034	
1035	Following the symlink
1036	---------------------
1037	
1038	The main loop in `link_path_walk()` iterates seamlessly over all
1039	components in the path and all of the non-final symlinks.  As symlinks
1040	are processed, the `name` pointer is adjusted to point to a new
1041	symlink, or is restored from the stack, so that much of the loop
1042	doesn't need to notice.  Getting this `name` variable on and off the
1043	stack is very straightforward; pushing and popping the references is
1044	a little more complex.
1045	
1046	When a symlink is found, `walk_component()` returns the value `1`
1047	(`0` is returned for any other sort of success, and a negative number
1048	is, as usual, an error indicator).  This causes `get_link()` to be
1049	called; it then gets the link from the filesystem.  Providing that
1050	operation is successful, the old path `name` is placed on the stack,
1051	and the new value is used as the `name` for a while.  When the end of
1052	the path is found (i.e. `*name` is `'\0'`) the old `name` is restored
1053	off the stack and path walking continues.
1054	
1055	Pushing and popping the reference pointers (inode, cookie, etc.) is more
1056	complex in part because of the desire to handle tail recursion.  When
1057	the last component of a symlink itself points to a symlink, we
1058	want to pop the symlink-just-completed off the stack before pushing
1059	the symlink-just-found to avoid leaving empty path remnants that would
1060	just get in the way.
1061	
1062	It is most convenient to push the new symlink references onto the
1063	stack in `walk_component()` immediately when the symlink is found;
1064	`walk_component()` is also the last piece of code that needs to look at the
1065	old symlink as it walks that last component.  So it is quite
1066	convenient for `walk_component()` to release the old symlink and pop
1067	the references just before pushing the reference information for the
1068	new symlink.  It is guided in this by two flags; `WALK_GET`, which
1069	gives it permission to follow a symlink if it finds one, and
1070	`WALK_PUT`, which tells it to release the current symlink after it has been
1071	followed.  `WALK_PUT` is tested first, leading to a call to
1072	`put_link()`.  `WALK_GET` is tested subsequently (by
1073	`should_follow_link()`) leading to a call to `pick_link()` which sets
1074	up the stack frame.
1075	
1076	### Symlinks with no final component ###
1077	
1078	A pair of special-case symlinks deserve a little further explanation.
1079	Both result in a new `struct path` (with mount and dentry) being set
1080	up in the `nameidata`, and result in `get_link()` returning `NULL`.
1081	
1082	The more obvious case is a symlink to "`/`".  All symlinks starting
1083	with "`/`" are detected in `get_link()` which resets the `nameidata`
1084	to point to the effective filesystem root.  If the symlink only
1085	contains "`/`" then there is nothing more to do, no components at all,
1086	so `NULL` is returned to indicate that the symlink can be released and
1087	the stack frame discarded.
1088	
1089	The other case involves things in `/proc` that look like symlinks but
1090	aren't really.
1091	
1092	>     $ ls -l /proc/self/fd/1
1093	>     lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
1094	
1095	Every open file descriptor in any process is represented in `/proc` by
1096	something that looks like a symlink.  It is really a reference to the
1097	target file, not just the name of it.  When you `readlink` these
1098	objects you get a name that might refer to the same file - unless it
1099	has been unlinked or mounted over.  When `walk_component()` follows
1100	one of these, the `->follow_link()` method in "procfs" doesn't return
1101	a string name, but instead calls `nd_jump_link()` which updates the
1102	`nameidata` in place to point to that target.  `->follow_link()` then
1103	returns `NULL`.  Again there is no final component and `get_link()`
1104	reports this by leaving the `last_type` field of `nameidata` as
1105	`LAST_BIND`.
1106	
1107	Following the symlink in the final component
1108	--------------------------------------------
1109	
1110	All this leads to `link_path_walk()` walking down every component, and
1111	following all symbolic links it finds, until it reaches the final
1112	component.  This is just returned in the `last` field of `nameidata`.
1113	For some callers, this is all they need; they want to create that
1114	`last` name if it doesn't exist or give an error if it does.  Other
1115	callers will want to follow a symlink if one is found, and possibly
1116	apply special handling to the last component of that symlink, rather
1117	than just the last component of the original file name.  These callers
1118	potentially need to call `link_path_walk()` again and again on
1119	successive symlinks until one is found that doesn't point to another
1120	symlink.
1121	
1122	This case is handled by the relevant caller of `link_path_walk()`, such as
1123	`path_lookupat()` using a loop that calls `link_path_walk()`, and then
1124	handles the final component.  If the final component is a symlink
1125	that needs to be followed, then `trailing_symlink()` is called to set
1126	things up properly and the loop repeats, calling `link_path_walk()`
1127	again.  This could loop as many as 40 times if the last component of
1128	each symlink is another symlink.
1129	
1130	The various functions that examine the final component and possibly
1131	report that it is a symlink are `lookup_last()`, `mountpoint_last()`
1132	and `do_last()`, each of which use the same convention as
1133	`walk_component()` of returning `1` if a symlink was found that needs
1134	to be followed.
1135	
1136	Of these, `do_last()` is the most interesting as it is used for
1137	opening a file.  Part of `do_last()` runs with `i_mutex` held and this
1138	part is in a separate function: `lookup_open()`.
1139	
1140	Explaining `do_last()` completely is beyond the scope of this article,
1141	but a few highlights should help those interested in exploring the
1142	code.
1143	
1144	1. Rather than just finding the target file, `do_last()` needs to open
1145	 it.  If the file was found in the dcache, then `vfs_open()` is used for
1146	 this.  If not, then `lookup_open()` will either call `atomic_open()` (if
1147	 the filesystem provides it) to combine the final lookup with the open, or
1148	 will perform the separate `lookup_real()` and `vfs_create()` steps
1149	 directly.  In the later case the actual "open" of this newly found or
1150	 created file will be performed by `vfs_open()`, just as if the name
1151	 were found in the dcache.
1152	
1153	2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information
1154	 wasn't quite current enough.  Rather than restarting the lookup from
1155	 the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead,
1156	 giving the filesystem a chance to resolve small inconsistencies.
1157	 If that doesn't work, only then is the lookup restarted from the top.
1158	
1159	3. An open with O_CREAT **does** follow a symlink in the final component,
1160	     unlike other creation system calls (like `mkdir`).  So the sequence:
1161	
1162	     >     ln -s bar /tmp/foo
1163	     >     echo hello > /tmp/foo
1164	
1165	     will create a file called `/tmp/bar`.  This is not permitted if
1166	     `O_EXCL` is set but otherwise is handled for an O_CREAT open much
1167	     like for a non-creating open: `should_follow_link()` returns `1`, and
1168	     so does `do_last()` so that `trailing_symlink()` gets called and the
1169	     open process continues on the symlink that was found.
1170	
1171	Updating the access time
1172	------------------------
1173	
1174	We previously said of RCU-walk that it would "take no locks, increment
1175	no counts, leave no footprints."  We have since seen that some
1176	"footprints" can be needed when handling symlinks as a counted
1177	reference (or even a memory allocation) may be needed.  But these
1178	footprints are best kept to a minimum.
1179	
1180	One other place where walking down a symlink can involve leaving
1181	footprints in a way that doesn't affect directories is in updating access times.
1182	In Unix (and Linux) every filesystem object has a "last accessed
1183	time", or "`atime`".  Passing through a directory to access a file
1184	within is not considered to be an access for the purposes of
1185	`atime`; only listing the contents of a directory can update its `atime`.
1186	Symlinks are different it seems.  Both reading a symlink (with `readlink()`)
1187	and looking up a symlink on the way to some other destination can
1188	update the atime on that symlink.
1189	
1190	[clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
1191	
1192	It is not clear why this is the case; POSIX has little to say on the
1193	subject.  The [clearest statement] is that, if a particular implementation
1194	updates a timestamp in a place not specified by POSIX, this must be
1195	documented "except that any changes caused by pathname resolution need
1196	not be documented".  This seems to imply that POSIX doesn't really
1197	care about access-time updates during pathname lookup.
1198	
1199	[Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
1200	
1201	An examination of history shows that prior to [Linux 1.3.87], the ext2
1202	filesystem, at least, didn't update atime when following a link.
1203	Unfortunately we have no record of why that behavior was changed.
1204	
1205	In any case, access time must now be updated and that operation can be
1206	quite complex.  Trying to stay in RCU-walk while doing it is best
1207	avoided.  Fortunately it is often permitted to skip the `atime`
1208	update.  Because `atime` updates cause performance problems in various
1209	areas, Linux supports the `relatime` mount option, which generally
1210	limits the updates of `atime` to once per day on files that aren't
1211	being changed (and symlinks never change once created).  Even without
1212	`relatime`, many filesystems record `atime` with a one-second
1213	granularity, so only one update per second is required.
1214	
1215	It is easy to test if an `atime` update is needed while in RCU-walk
1216	mode and, if it isn't, the update can be skipped and RCU-walk mode
1217	continues.  Only when an `atime` update is actually required does the
1218	path walk drop down to REF-walk.  All of this is handled in the
1219	`get_link()` function.
1220	
1221	A few flags
1222	-----------
1223	
1224	A suitable way to wrap up this tour of pathname walking is to list
1225	the various flags that can be stored in the `nameidata` to guide the
1226	lookup process.  Many of these are only meaningful on the final
1227	component, others reflect the current state of the pathname lookup.
1228	And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with
1229	the others.  If this is not set, an empty pathname causes an error
1230	very early on.  If it is set, empty pathnames are not considered to be
1231	an error.
1232	
1233	### Global state flags ###
1234	
1235	We have already met two global state flags: `LOOKUP_RCU` and
1236	`LOOKUP_REVAL`.  These select between one of three overall approaches
1237	to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
1238	
1239	`LOOKUP_PARENT` indicates that the final component hasn't been reached
1240	yet.  This is primarily used to tell the audit subsystem the full
1241	context of a particular access being audited.
1242	
1243	`LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was
1244	provided by the caller, so it shouldn't be released when it is no
1245	longer needed.
1246	
1247	`LOOKUP_JUMPED` means that the current dentry was chosen not because
1248	it had the right name but for some other reason.  This happens when
1249	following "`..`", following a symlink to `/`, crossing a mount point
1250	or accessing a "`/proc/$PID/fd/$FD`" symlink.  In this case the
1251	filesystem has not been asked to revalidate the name (with
1252	`d_revalidate()`).  In such cases the inode may still need to be
1253	revalidated, so `d_op->d_weak_revalidate()` is called if
1254	`LOOKUP_JUMPED` is set when the look completes - which may be at the
1255	final component or, when creating, unlinking, or renaming, at the penultimate component.
1256	
1257	### Final-component flags ###
1258	
1259	Some of these flags are only set when the final component is being
1260	considered.  Others are only checked for when considering that final
1261	component.
1262	
1263	`LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount
1264	point, then the mount is triggered.  Some operations would trigger it
1265	anyway, but operations like `stat()` deliberately don't.  `statfs()`
1266	needs to trigger the mount but otherwise behaves a lot like `stat()`, so
1267	it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of
1268	"`mount --bind`".
1269	
1270	`LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for
1271	symlinks.  Some system calls set or clear it implicitly, while
1272	others have API flags such as `AT_SYMLINK_FOLLOW` and
1273	`UMOUNT_NOFOLLOW` to control it.  Its effect is similar to
1274	`WALK_GET` that we already met, but it is used in a different way.
1275	
1276	`LOOKUP_DIRECTORY` insists that the final component is a directory.
1277	Various callers set this and it is also set when the final component
1278	is found to be followed by a slash.
1279	
1280	Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and
1281	`LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made
1282	available to the filesystem and particularly the `->d_revalidate()`
1283	method.  A filesystem can choose not to bother revalidating too hard
1284	if it knows that it will be asked to open or create the file soon.
1285	These flags were previously useful for `->lookup()` too but with the
1286	introduction of `->atomic_open()` they are less relevant there.
1287	
1288	End of the road
1289	---------------
1290	
1291	Despite its complexity, all this pathname lookup code appears to be
1292	in good shape - various parts are certainly easier to understand now
1293	than even a couple of releases ago.  But that doesn't mean it is
1294	"finished".   As already mentioned, RCU-walk currently only follows
1295	symlinks that are stored in the inode so, while it handles many ext4
1296	symlinks, it doesn't help with NFS, XFS, or Btrfs.  That support
1297	is not likely to be long delayed.
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