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diff --git a/Documentation/filesystems/path-lookup.md b/Documentation/filesystems/path-lookup.md deleted file mode 100644 index e2edd45c4bc0..000000000000 --- a/Documentation/filesystems/path-lookup.md +++ /dev/null @@ -1,1297 +0,0 @@ -<head> -<style> p { max-width:50em} ol, ul {max-width: 40em}</style> -</head> - -Pathname lookup in Linux. -========================= - -This write-up is based on three articles published at lwn.net: - -- <https://lwn.net/Articles/649115/> Pathname lookup in Linux -- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux -- <https://lwn.net/Articles/650786/> A walk among the symlinks - -Written by Neil Brown with help from Al Viro and Jon Corbet. - -Introduction ------------- - -The most obvious aspect of pathname lookup, which very little -exploration is needed to discover, is that it is complex. There are -many rules, special cases, and implementation alternatives that all -combine to confuse the unwary reader. Computer science has long been -acquainted with such complexity and has tools to help manage it. One -tool that we will make extensive use of is "divide and conquer". For -the early parts of the analysis we will divide off symlinks - leaving -them until the final part. Well before we get to symlinks we have -another major division based on the VFS's approach to locking which -will allow us to review "REF-walk" and "RCU-walk" separately. But we -are getting ahead of ourselves. There are some important low level -distinctions we need to clarify first. - -There are two sorts of ... --------------------------- - -[`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html - -Pathnames (sometimes "file names"), used to identify objects in the -filesystem, will be familiar to most readers. They contain two sorts -of elements: "slashes" that are sequences of one or more "`/`" -characters, and "components" that are sequences of one or more -non-"`/`" characters. These form two kinds of paths. Those that -start with slashes are "absolute" and start from the filesystem root. -The others are "relative" and start from the current directory, or -from some other location specified by a file descriptor given to a -"xxx`at`" system call such as "[`openat()`]". - -[`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html - -It is tempting to describe the second kind as starting with a -component, but that isn't always accurate: a pathname can lack both -slashes and components, it can be empty, in other words. This is -generally forbidden in POSIX, but some of those "xxx`at`" system calls -in Linux permit it when the `AT_EMPTY_PATH` flag is given. For -example, if you have an open file descriptor on an executable file you -can execute it by calling [`execveat()`] passing the file descriptor, -an empty path, and the `AT_EMPTY_PATH` flag. - -These paths can be divided into two sections: the final component and -everything else. The "everything else" is the easy bit. In all cases -it must identify a directory that already exists, otherwise an error -such as `ENOENT` or `ENOTDIR` will be reported. - -The final component is not so simple. Not only do different system -calls interpret it quite differently (e.g. some create it, some do -not), but it might not even exist: neither the empty pathname nor the -pathname that is just slashes have a final component. If it does -exist, it could be "`.`" or "`..`" which are handled quite differently -from other components. - -[POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12 - -If a pathname ends with a slash, such as "`/tmp/foo/`" it might be -tempting to consider that to have an empty final component. In many -ways that would lead to correct results, but not always. In -particular, `mkdir()` and `rmdir()` each create or remove a directory named -by the final component, and they are required to work with pathnames -ending in "`/`". According to [POSIX] - -> A pathname that contains at least one non- <slash> character and -> that ends with one or more trailing <slash> characters shall not -> be resolved successfully unless the last pathname component before -> the trailing <slash> characters names an existing directory or a -> directory entry that is to be created for a directory immediately -> after the pathname is resolved. - -The Linux pathname walking code (mostly in `fs/namei.c`) deals with -all of these issues: breaking the path into components, handling the -"everything else" quite separately from the final component, and -checking that the trailing slash is not used where it isn't -permitted. It also addresses the important issue of concurrent -access. - -While one process is looking up a pathname, another might be making -changes that affect that lookup. One fairly extreme case is that if -"a/b" were renamed to "a/c/b" while another process were looking up -"a/b/..", that process might successfully resolve on "a/c". -Most races are much more subtle, and a big part of the task of -pathname lookup is to prevent them from having damaging effects. Many -of the possible races are seen most clearly in the context of the -"dcache" and an understanding of that is central to understanding -pathname lookup. - -More than just a cache. ------------------------ - -The "dcache" caches information about names in each filesystem to -make them quickly available for lookup. Each entry (known as a -"dentry") contains three significant fields: a component name, a -pointer to a parent dentry, and a pointer to the "inode" which -contains further information about the object in that parent with -the given name. The inode pointer can be `NULL` indicating that the -name doesn't exist in the parent. While there can be linkage in the -dentry of a directory to the dentries of the children, that linkage is -not used for pathname lookup, and so will not be considered here. - -The dcache has a number of uses apart from accelerating lookup. One -that will be particularly relevant is that it is closely integrated -with the mount table that records which filesystem is mounted where. -What the mount table actually stores is which dentry is mounted on top -of which other dentry. - -When considering the dcache, we have another of our "two types" -distinctions: there are two types of filesystems. - -Some filesystems ensure that the information in the dcache is always -completely accurate (though not necessarily complete). This can allow -the VFS to determine if a particular file does or doesn't exist -without checking with the filesystem, and means that the VFS can -protect the filesystem against certain races and other problems. -These are typically "local" filesystems such as ext3, XFS, and Btrfs. - -Other filesystems don't provide that guarantee because they cannot. -These are typically filesystems that are shared across a network, -whether remote filesystems like NFS and 9P, or cluster filesystems -like ocfs2 or cephfs. These filesystems allow the VFS to revalidate -cached information, and must provide their own protection against -awkward races. The VFS can detect these filesystems by the -`DCACHE_OP_REVALIDATE` flag being set in the dentry. - -REF-walk: simple concurrency management with refcounts and spinlocks --------------------------------------------------------------------- - -With all of those divisions carefully classified, we can now start -looking at the actual process of walking along a path. In particular -we will start with the handling of the "everything else" part of a -pathname, and focus on the "REF-walk" approach to concurrency -management. This code is found in the `link_path_walk()` function, if -you ignore all the places that only run when "`LOOKUP_RCU`" -(indicating the use of RCU-walk) is set. - -[Meet the Lockers]: https://lwn.net/Articles/453685/ - -REF-walk is fairly heavy-handed with locks and reference counts. Not -as heavy-handed as in the old "big kernel lock" days, but certainly not -afraid of taking a lock when one is needed. It uses a variety of -different concurrency controls. A background understanding of the -various primitives is assumed, or can be gleaned from elsewhere such -as in [Meet the Lockers]. - -The locking mechanisms used by REF-walk include: - -### dentry->d_lockref ### - -This uses the lockref primitive to provide both a spinlock and a -reference count. The special-sauce of this primitive is that the -conceptual sequence "lock; inc_ref; unlock;" can often be performed -with a single atomic memory operation. - -Holding a reference on a dentry ensures that the dentry won't suddenly -be freed and used for something else, so the values in various fields -will behave as expected. It also protects the `->d_inode` reference -to the inode to some extent. - -The association between a dentry and its inode is fairly permanent. -For example, when a file is renamed, the dentry and inode move -together to the new location. When a file is created the dentry will -initially be negative (i.e. `d_inode` is `NULL`), and will be assigned -to the new inode as part of the act of creation. - -When a file is deleted, this can be reflected in the cache either by -setting `d_inode` to `NULL`, or by removing it from the hash table -(described shortly) used to look up the name in the parent directory. -If the dentry is still in use the second option is used as it is -perfectly legal to keep using an open file after it has been deleted -and having the dentry around helps. If the dentry is not otherwise in -use (i.e. if the refcount in `d_lockref` is one), only then will -`d_inode` be set to `NULL`. Doing it this way is more efficient for a -very common case. - -So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode` -value will never be changed. - -### dentry->d_lock ### - -`d_lock` is a synonym for the spinlock that is part of `d_lockref` above. -For our purposes, holding this lock protects against the dentry being -renamed or unlinked. In particular, its parent (`d_parent`), and its -name (`d_name`) cannot be changed, and it cannot be removed from the -dentry hash table. - -When looking for a name in a directory, REF-walk takes `d_lock` on -each candidate dentry that it finds in the hash table and then checks -that the parent and name are correct. So it doesn't lock the parent -while searching in the cache; it only locks children. - -When looking for the parent for a given name (to handle "`..`"), -REF-walk can take `d_lock` to get a stable reference to `d_parent`, -but it first tries a more lightweight approach. As seen in -`dget_parent()`, if a reference can be claimed on the parent, and if -subsequently `d_parent` can be seen to have not changed, then there is -no need to actually take the lock on the child. - -### rename_lock ### - -Looking up a given name in a given directory involves computing a hash -from the two values (the name and the dentry of the directory), -accessing that slot in a hash table, and searching the linked list -that is found there. - -When a dentry is renamed, the name and the parent dentry can both -change so the hash will almost certainly change too. This would move the -dentry to a different chain in the hash table. If a filename search -happened to be looking at a dentry that was moved in this way, -it might end up continuing the search down the wrong chain, -and so miss out on part of the correct chain. - -The name-lookup process (`d_lookup()`) does _not_ try to prevent this -from happening, but only to detect when it happens. -`rename_lock` is a seqlock that is updated whenever any dentry is -renamed. If `d_lookup` finds that a rename happened while it -unsuccessfully scanned a chain in the hash table, it simply tries -again. - -### inode->i_mutex ### - -`i_mutex` is a mutex that serializes all changes to a particular -directory. This ensures that, for example, an `unlink()` and a `rename()` -cannot both happen at the same time. It also keeps the directory -stable while the filesystem is asked to look up a name that is not -currently in the dcache. - -This has a complementary role to that of `d_lock`: `i_mutex` on a -directory protects all of the names in that directory, while `d_lock` -on a name protects just one name in a directory. Most changes to the -dcache hold `i_mutex` on the relevant directory inode and briefly take -`d_lock` on one or more the dentries while the change happens. One -exception is when idle dentries are removed from the dcache due to -memory pressure. This uses `d_lock`, but `i_mutex` plays no role. - -The mutex affects pathname lookup in two distinct ways. Firstly it -serializes lookup of a name in a directory. `walk_component()` uses -`lookup_fast()` first which, in turn, checks to see if the name is in the cache, -using only `d_lock` locking. If the name isn't found, then `walk_component()` -falls back to `lookup_slow()` which takes `i_mutex`, checks again that -the name isn't in the cache, and then calls in to the filesystem to get a -definitive answer. A new dentry will be added to the cache regardless of -the result. - -Secondly, when pathname lookup reaches the final component, it will -sometimes need to take `i_mutex` before performing the last lookup so -that the required exclusion can be achieved. How path lookup chooses -to take, or not take, `i_mutex` is one of the -issues addressed in a subsequent section. - -### mnt->mnt_count ### - -`mnt_count` is a per-CPU reference counter on "`mount`" structures. -Per-CPU here means that incrementing the count is cheap as it only -uses CPU-local memory, but checking if the count is zero is expensive as -it needs to check with every CPU. Taking a `mnt_count` reference -prevents the mount structure from disappearing as the result of regular -unmount operations, but does not prevent a "lazy" unmount. So holding -`mnt_count` doesn't ensure that the mount remains in the namespace and, -in particular, doesn't stabilize the link to the mounted-on dentry. It -does, however, ensure that the `mount` data structure remains coherent, -and it provides a reference to the root dentry of the mounted -filesystem. So a reference through `->mnt_count` provides a stable -reference to the mounted dentry, but not the mounted-on dentry. - -### mount_lock ### - -`mount_lock` is a global seqlock, a bit like `rename_lock`. It can be used to -check if any change has been made to any mount points. - -While walking down the tree (away from the root) this lock is used when -crossing a mount point to check that the crossing was safe. That is, -the value in the seqlock is read, then the code finds the mount that -is mounted on the current directory, if there is one, and increments -the `mnt_count`. Finally the value in `mount_lock` is checked against -the old value. If there is no change, then the crossing was safe. If there -was a change, the `mnt_count` is decremented and the whole process is -retried. - -When walking up the tree (towards the root) by following a ".." link, -a little more care is needed. In this case the seqlock (which -contains both a counter and a spinlock) is fully locked to prevent -any changes to any mount points while stepping up. This locking is -needed to stabilize the link to the mounted-on dentry, which the -refcount on the mount itself doesn't ensure. - -### RCU ### - -Finally the global (but extremely lightweight) RCU read lock is held -from time to time to ensure certain data structures don't get freed -unexpectedly. - -In particular it is held while scanning chains in the dcache hash -table, and the mount point hash table. - -Bringing it together with `struct nameidata` --------------------------------------------- - -[First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s - -Throughout the process of walking a path, the current status is stored -in a `struct nameidata`, "namei" being the traditional name - dating -all the way back to [First Edition Unix] - of the function that -converts a "name" to an "inode". `struct nameidata` contains (among -other fields): - -### `struct path path` ### - -A `path` contains a `struct vfsmount` (which is -embedded in a `struct mount`) and a `struct dentry`. Together these -record the current status of the walk. They start out referring to the -starting point (the current working directory, the root directory, or some other -directory identified by a file descriptor), and are updated on each -step. A reference through `d_lockref` and `mnt_count` is always -held. - -### `struct qstr last` ### - -This is a string together with a length (i.e. _not_ `nul` terminated) -that is the "next" component in the pathname. - -### `int last_type` ### - -This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or -`LAST_BIND`. The `last` field is only valid if the type is -`LAST_NORM`. `LAST_BIND` is used when following a symlink and no -components of the symlink have been processed yet. Others should be -fairly self-explanatory. - -### `struct path root` ### - -This is used to hold a reference to the effective root of the -filesystem. Often that reference won't be needed, so this field is -only assigned the first time it is used, or when a non-standard root -is requested. Keeping a reference in the `nameidata` ensures that -only one root is in effect for the entire path walk, even if it races -with a `chroot()` system call. - -The root is needed when either of two conditions holds: (1) either the -pathname or a symbolic link starts with a "'/'", or (2) a "`..`" -component is being handled, since "`..`" from the root must always stay -at the root. The value used is usually the current root directory of -the calling process. An alternate root can be provided as when -`sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call -`mount_subtree()`. In each case a pathname is being looked up in a very -specific part of the filesystem, and the lookup must not be allowed to -escape that subtree. It works a bit like a local `chroot()`. - -Ignoring the handling of symbolic links, we can now describe the -"`link_path_walk()`" function, which handles the lookup of everything -except the final component as: - -> Given a path (`name`) and a nameidata structure (`nd`), check that the -> current directory has execute permission and then advance `name` -> over one component while updating `last_type` and `last`. If that -> was the final component, then return, otherwise call -> `walk_component()` and repeat from the top. - -`walk_component()` is even easier. If the component is `LAST_DOTS`, -it calls `handle_dots()` which does the necessary locking as already -described. If it finds a `LAST_NORM` component it first calls -"`lookup_fast()`" which only looks in the dcache, but will ask the -filesystem to revalidate the result if it is that sort of filesystem. -If that doesn't get a good result, it calls "`lookup_slow()`" which -takes the `i_mutex`, rechecks the cache, and then asks the filesystem -to find a definitive answer. Each of these will call -`follow_managed()` (as described below) to handle any mount points. - -In the absence of symbolic links, `walk_component()` creates a new -`struct path` containing a counted reference to the new dentry and a -reference to the new `vfsmount` which is only counted if it is -different from the previous `vfsmount`. It then calls -`path_to_nameidata()` to install the new `struct path` in the -`struct nameidata` and drop the unneeded references. - -This "hand-over-hand" sequencing of getting a reference to the new -dentry before dropping the reference to the previous dentry may -seem obvious, but is worth pointing out so that we will recognize its -analogue in the "RCU-walk" version. - -Handling the final component. ------------------------------ - -`link_path_walk()` only walks as far as setting `nd->last` and -`nd->last_type` to refer to the final component of the path. It does -not call `walk_component()` that last time. Handling that final -component remains for the caller to sort out. Those callers are -`path_lookupat()`, `path_parentat()`, `path_mountpoint()` and -`path_openat()` each of which handles the differing requirements of -different system calls. - -`path_parentat()` is clearly the simplest - it just wraps a little bit -of housekeeping around `link_path_walk()` and returns the parent -directory and final component to the caller. The caller will be either -aiming to create a name (via `filename_create()`) or remove or rename -a name (in which case `user_path_parent()` is used). They will use -`i_mutex` to exclude other changes while they validate and then -perform their operation. - -`path_lookupat()` is nearly as simple - it is used when an existing -object is wanted such as by `stat()` or `chmod()`. It essentially just -calls `walk_component()` on the final component through a call to -`lookup_last()`. `path_lookupat()` returns just the final dentry. - -`path_mountpoint()` handles the special case of unmounting which must -not try to revalidate the mounted filesystem. It effectively -contains, through a call to `mountpoint_last()`, an alternate -implementation of `lookup_slow()` which skips that step. This is -important when unmounting a filesystem that is inaccessible, such as -one provided by a dead NFS server. - -Finally `path_openat()` is used for the `open()` system call; it -contains, in support functions starting with "`do_last()`", all the -complexity needed to handle the different subtleties of O_CREAT (with -or without O_EXCL), final "`/`" characters, and trailing symbolic -links. We will revisit this in the final part of this series, which -focuses on those symbolic links. "`do_last()`" will sometimes, but -not always, take `i_mutex`, depending on what it finds. - -Each of these, or the functions which call them, need to be alert to -the possibility that the final component is not `LAST_NORM`. If the -goal of the lookup is to create something, then any value for -`last_type` other than `LAST_NORM` will result in an error. For -example if `path_parentat()` reports `LAST_DOTDOT`, then the caller -won't try to create that name. They also check for trailing slashes -by testing `last.name[last.len]`. If there is any character beyond -the final component, it must be a trailing slash. - -Revalidation and automounts ---------------------------- - -Apart from symbolic links, there are only two parts of the "REF-walk" -process not yet covered. One is the handling of stale cache entries -and the other is automounts. - -On filesystems that require it, the lookup routines will call the -`->d_revalidate()` dentry method to ensure that the cached information -is current. This will often confirm validity or update a few details -from a server. In some cases it may find that there has been change -further up the path and that something that was thought to be valid -previously isn't really. When this happens the lookup of the whole -path is aborted and retried with the "`LOOKUP_REVAL`" flag set. This -forces revalidation to be more thorough. We will see more details of -this retry process in the next article. - -Automount points are locations in the filesystem where an attempt to -lookup a name can trigger changes to how that lookup should be -handled, in particular by mounting a filesystem there. These are -covered in greater detail in 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_mutex` or even `rename_lock` ### - -A mutex is a fairly heavyweight lock that can only be taken when it is -permissible to sleep. As `rcu_read_lock()` forbids sleeping, -`inode->i_mutex` plays no role in RCU-walk. If some other thread does -take `i_mutex` and modifies the directory in a way that RCU-walk needs -to notice, the result will be either that RCU-walk fails to find the -dentry that it is looking for, or it will find a dentry which -`read_seqretry()` won't validate. In either case it will drop down to -REF-walk mode which can take whatever locks are needed. - -Though `rename_lock` could be used by RCU-walk as it doesn't require -any sleeping, RCU-walk doesn't bother. REF-walk uses `rename_lock` to -protect against the possibility of hash chains in the dcache changing -while they are being searched. This can result in failing to find -something that actually is there. When RCU-walk fails to find -something in the dentry cache, whether it is really there or not, it -already drops down to REF-walk and tries again with appropriate -locking. This neatly handles all cases, so adding extra checks on -rename_lock would bring no significant value. - -`unlazy walk()` and `complete_walk()` -------------------------------------- - -That "dropping down to REF-walk" typically involves a call to -`unlazy_walk()`, so named because "RCU-walk" is also sometimes -referred to as "lazy walk". `unlazy_walk()` is called when -following the path down to the current vfsmount/dentry pair seems to -have proceeded successfully, but the next step is problematic. This -can happen if the next name cannot be found in the dcache, if -permission checking or name revalidation couldn't be achieved while -the `rcu_read_lock()` is held (which forbids sleeping), if an -automount point is found, or in a couple of cases involving symlinks. -It is also called from `complete_walk()` when the lookup has reached -the final component, or the very end of the path, depending on which -particular flavor of lookup is used. - -Other reasons for dropping out of RCU-walk that do not trigger a call -to `unlazy_walk()` are when some inconsistency is found that cannot be -handled immediately, such as `mount_lock` or one of the `d_seq` -seqlocks reporting a change. In these cases the relevant function -will return `-ECHILD` which will percolate up until it triggers a new -attempt from the top using REF-walk. - -For those cases where `unlazy_walk()` is an option, it essentially -takes a reference on each of the pointers that it holds (vfsmount, -dentry, and possibly some symbolic links) and then verifies that the -relevant seqlocks have not been changed. If there have been changes, -it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk -has been a success and the lookup process continues. - -Taking a reference on those pointers is not quite as simple as just -incrementing a counter. That works to take a second reference if you -already have one (often indirectly through another object), but it -isn't sufficient if you don't actually have a counted reference at -all. For `dentry->d_lockref`, it is safe to increment the reference -counter to get a reference unless it has been explicitly marked as -"dead" which involves setting the counter to `-128`. -`lockref_get_not_dead()` achieves this. - -For `mnt->mnt_count` it is safe to take a reference as long as -`mount_lock` is then used to validate the reference. If that -validation fails, it may *not* be safe to just drop that reference in -the standard way of calling `mnt_put()` - an unmount may have -progressed too far. So the code in `legitimize_mnt()`, when it -finds that the reference it got might not be safe, checks the -`MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is -correct, or if it should just decrement the count and pretend none of -this ever happened. - -Taking care in filesystems ---------------------------- - -RCU-walk depends almost entirely on cached information and often will -not call into the filesystem at all. However there are two places, -besides the already-mentioned component-name comparison, where the -file system might be included in RCU-walk, and it must know to be -careful. - -If the filesystem has non-standard permission-checking requirements - -such as a networked filesystem which may need to check with the server -- the `i_op->permission` interface might be called during RCU-walk. -In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it -knows not to sleep, but to return `-ECHILD` if it cannot complete -promptly. `i_op->permission` is given the inode pointer, not the -dentry, so it doesn't need to worry about further consistency checks. -However if it accesses any other filesystem data structures, it must -ensure they are safe to be accessed with only the `rcu_read_lock()` -held. This typically means they must be freed using `kfree_rcu()` or -similar. - -[`READ_ONCE()`]: https://lwn.net/Articles/624126/ - -If the filesystem may need to revalidate dcache entries, then -`d_op->d_revalidate` may be called in RCU-walk too. This interface -*is* passed the dentry but does not have access to the `inode` or the -`seq` number from the `nameidata`, so it needs to be extra careful -when accessing fields in the dentry. This "extra care" typically -involves using [`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. - -> $ ls -l /proc/self/fd/1 -> lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4 - -Every open file descriptor in any process is represented in `/proc` by -something that looks like a symlink. It is really a reference to the -target file, not just the name of it. When you `readlink` these -objects you get a name that might refer to the same file - unless it -has been unlinked or mounted over. When `walk_component()` follows -one of these, the `->follow_link()` method in "procfs" doesn't return -a string name, but instead calls `nd_jump_link()` which updates the -`nameidata` in place to point to that target. `->follow_link()` then -returns `NULL`. Again there is no final component and `get_link()` -reports this by leaving the `last_type` field of `nameidata` as -`LAST_BIND`. - -Following the symlink in the final component --------------------------------------------- - -All this leads to `link_path_walk()` walking down every component, and -following all symbolic links it finds, until it reaches the final -component. This is just returned in the `last` field of `nameidata`. -For some callers, this is all they need; they want to create that -`last` name if it doesn't exist or give an error if it does. Other -callers will want to follow a symlink if one is found, and possibly -apply special handling to the last component of that symlink, rather -than just the last component of the original file name. These callers -potentially need to call `link_path_walk()` again and again on -successive symlinks until one is found that doesn't point to another -symlink. - -This case is handled by the relevant caller of `link_path_walk()`, such as -`path_lookupat()` using a loop that calls `link_path_walk()`, and then -handles the final component. If the final component is a symlink -that needs to be followed, then `trailing_symlink()` is called to set -things up properly and the loop repeats, calling `link_path_walk()` -again. This could loop as many as 40 times if the last component of -each symlink is another symlink. - -The various functions that examine the final component and possibly -report that it is a symlink are `lookup_last()`, `mountpoint_last()` -and `do_last()`, each of which use the same convention as -`walk_component()` of returning `1` if a symlink was found that needs -to be followed. - -Of these, `do_last()` is the most interesting as it is used for -opening a file. Part of `do_last()` runs with `i_mutex` held and this -part is in a separate function: `lookup_open()`. - -Explaining `do_last()` completely is beyond the scope of this article, -but a few highlights should help those interested in exploring the -code. - -1. Rather than just finding the target file, `do_last()` needs to open - it. If the file was found in the dcache, then `vfs_open()` is used for - this. If not, then `lookup_open()` will either call `atomic_open()` (if - the filesystem provides it) to combine the final lookup with the open, or - will perform the separate `lookup_real()` and `vfs_create()` steps - directly. In the later case the actual "open" of this newly found or - created file will be performed by `vfs_open()`, just as if the name - were found in the dcache. - -2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information - wasn't quite current enough. Rather than restarting the lookup from - the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead, - giving the filesystem a chance to resolve small inconsistencies. - If that doesn't work, only then is the lookup restarted from the top. - -3. An open with O_CREAT **does** follow a symlink in the final component, - unlike other creation system calls (like `mkdir`). So the sequence: - - > ln -s bar /tmp/foo - > echo hello > /tmp/foo - - will create a file called `/tmp/bar`. This is not permitted if - `O_EXCL` is set but otherwise is handled for an O_CREAT open much - like for a non-creating open: `should_follow_link()` returns `1`, and - so does `do_last()` so that `trailing_symlink()` gets called and the - open process continues on the symlink that was found. - -Updating the access time ------------------------- - -We previously said of RCU-walk that it would "take no locks, increment -no counts, leave no footprints." We have since seen that some -"footprints" can be needed when handling symlinks as a counted -reference (or even a memory allocation) may be needed. But these -footprints are best kept to a minimum. - -One other place where walking down a symlink can involve leaving -footprints in a way that doesn't affect directories is in updating access times. -In Unix (and Linux) every filesystem object has a "last accessed -time", or "`atime`". Passing through a directory to access a file -within is not considered to be an access for the purposes of -`atime`; only listing the contents of a directory can update its `atime`. -Symlinks are different it seems. Both reading a symlink (with `readlink()`) -and looking up a symlink on the way to some other destination can -update the atime on that symlink. - -[clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08 - -It is not clear why this is the case; POSIX has little to say on the -subject. The [clearest statement] is that, if a particular implementation -updates a timestamp in a place not specified by POSIX, this must be -documented "except that any changes caused by pathname resolution need -not be documented". This seems to imply that POSIX doesn't really -care about access-time updates during pathname lookup. - -[Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8 - -An examination of history shows that prior to [Linux 1.3.87], the ext2 -filesystem, at least, didn't update atime when following a link. -Unfortunately we have no record of why that behavior was changed. - -In any case, access time must now be updated and that operation can be -quite complex. Trying to stay in RCU-walk while doing it is best -avoided. Fortunately it is often permitted to skip the `atime` -update. Because `atime` updates cause performance problems in various -areas, Linux supports the `relatime` mount option, which generally -limits the updates of `atime` to once per day on files that aren't -being changed (and symlinks never change once created). Even without -`relatime`, many filesystems record `atime` with a one-second -granularity, so only one update per second is required. - -It is easy to test if an `atime` update is needed while in RCU-walk -mode and, if it isn't, the update can be skipped and RCU-walk mode -continues. Only when an `atime` update is actually required does the -path walk drop down to REF-walk. All of this is handled in the -`get_link()` function. - -A few flags ------------ - -A suitable way to wrap up this tour of pathname walking is to list -the various flags that can be stored in the `nameidata` to guide the -lookup process. Many of these are only meaningful on the final -component, others reflect the current state of the pathname lookup. -And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with -the others. If this is not set, an empty pathname causes an error -very early on. If it is set, empty pathnames are not considered to be -an error. - -### Global state flags ### - -We have already met two global state flags: `LOOKUP_RCU` and -`LOOKUP_REVAL`. These select between one of three overall approaches -to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation. - -`LOOKUP_PARENT` indicates that the final component hasn't been reached -yet. This is primarily used to tell the audit subsystem the full -context of a particular access being audited. - -`LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was -provided by the caller, so it shouldn't be released when it is no -longer needed. - -`LOOKUP_JUMPED` means that the current dentry was chosen not because -it had the right name but for some other reason. This happens when -following "`..`", following a symlink to `/`, crossing a mount point -or accessing a "`/proc/$PID/fd/$FD`" symlink. In this case the -filesystem has not been asked to revalidate the name (with -`d_revalidate()`). In such cases the inode may still need to be -revalidated, so `d_op->d_weak_revalidate()` is called if -`LOOKUP_JUMPED` is set when the look completes - which may be at the -final component or, when creating, unlinking, or renaming, at the penultimate component. - -### Final-component flags ### - -Some of these flags are only set when the final component is being -considered. Others are only checked for when considering that final -component. - -`LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount -point, then the mount is triggered. Some operations would trigger it -anyway, but operations like `stat()` deliberately don't. `statfs()` -needs to trigger the mount but otherwise behaves a lot like `stat()`, so -it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of -"`mount --bind`". - -`LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for -symlinks. Some system calls set or clear it implicitly, while -others have API flags such as `AT_SYMLINK_FOLLOW` and -`UMOUNT_NOFOLLOW` to control it. Its effect is similar to -`WALK_GET` that we already met, but it is used in a different way. - -`LOOKUP_DIRECTORY` insists that the final component is a directory. -Various callers set this and it is also set when the final component -is found to be followed by a slash. - -Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and -`LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made -available to the filesystem and particularly the `->d_revalidate()` -method. A filesystem can choose not to bother revalidating too hard -if it knows that it will be asked to open or create the file soon. -These flags were previously useful for `->lookup()` too but with the -introduction of `->atomic_open()` they are less relevant there. - -End of the road ---------------- - -Despite its complexity, all this pathname lookup code appears to be -in good shape - various parts are certainly easier to understand now -than even a couple of releases ago. But that doesn't mean it is -"finished". As already mentioned, RCU-walk currently only follows -symlinks that are stored in the inode so, while it handles many ext4 -symlinks, it doesn't help with NFS, XFS, or Btrfs. That support -is not likely to be long delayed. |