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fs: clarify when the i_version counter must be updated

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The i_version field in the kernel has had different semantics over
the decades, but NFSv4 has certain expectations. Update the comments
in iversion.h to describe when the i_version must change.

Cc: Colin Walters <walters@verbum.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Trond Myklebust <trondmy@hammerspace.com>
Cc: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
This commit is contained in:
Jeff Layton
2022-08-22 09:04:04 -04:00
parent c5bc1b3ff3
commit a3bb710383
1 changed files with 20 additions and 2 deletions
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22
include/linux/iversion.h
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@@ -9,8 +9,26 @@
* ---------------------------
* The change attribute (i_version) is mandated by NFSv4 and is mostly for
* knfsd, but is also used for other purposes (e.g. IMA). The i_version must
* appear different to observers if there was a change to the inode's data or
* metadata since it was last queried.
* appear larger to observers if there was an explicit change to the inode's
* data or metadata since it was last queried.
*
* An explicit change is one that would ordinarily result in a change to the
* inode status change time (aka ctime). i_version must appear to change, even
* if the ctime does not (since the whole point is to avoid missing updates due
* to timestamp granularity). If POSIX or other relevant spec mandates that the
* ctime must change due to an operation, then the i_version counter must be
* incremented as well.
*
* Making the i_version update completely atomic with the operation itself would
* be prohibitively expensive. Traditionally the kernel has updated the times on
* directories after an operation that changes its contents. For regular files,
* the ctime is usually updated before the data is copied into the cache for a
* write. This means that there is a window of time when an observer can
* associate a new timestamp with old file contents. Since the purpose of the
* i_version is to allow for better cache coherency, the i_version must always
* be updated after the results of the operation are visible. Updating it before
* and after a change is also permitted. (Note that no filesystems currently do
* this. Fixing that is a work-in-progress).
*
* Observers see the i_version as a 64-bit number that never decreases. If it
* remains the same since it was last checked, then nothing has changed in the