Documentation: add a new file documenting multigrain timestamps

JIRA: https://issues.redhat.com/browse/RHEL-121527

Add a high-level document that describes how multigrain timestamps work,
rationale for them, and some info about implementation and tradeoffs.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241002-mgtime-v10-8-d1c4717f5284@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
(cherry picked from commit e3fad03)
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>

Author: cmaiolino

Documentation/filesystems/index.rst

@@ -29,6 +29,7 @@ algorithms work.
    fiemap
    files
    locks
+   multigrain-ts
    mount_api
    quota
    seq_file

Documentation/filesystems/multigrain-ts.rst

@@ -0,0 +1,125 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Multigrain Timestamps
+=====================
+
+Introduction
+============
+Historically, the kernel has always used coarse time values to stamp inodes.
+This value is updated every jiffy, so any change that happens within that jiffy
+will end up with the same timestamp.
+
+When the kernel goes to stamp an inode (due to a read or write), it first gets
+the current time and then compares it to the existing timestamp(s) to see
+whether anything will change. If nothing changed, then it can avoid updating
+the inode's metadata.
+
+Coarse timestamps are therefore good from a performance standpoint, since they
+reduce the need for metadata updates, but bad from the standpoint of
+determining whether anything has changed, since a lot of things can happen in a
+jiffy.
+
+They are particularly troublesome with NFSv3, where unchanging timestamps can
+make it difficult to tell whether to invalidate caches. NFSv4 provides a
+dedicated change attribute that should always show a visible change, but not
+all filesystems implement this properly, causing the NFS server to substitute
+the ctime in many cases.
+
+Multigrain timestamps aim to remedy this by selectively using fine-grained
+timestamps when a file has had its timestamps queried recently, and the current
+coarse-grained time does not cause a change.
+
+Inode Timestamps
+================
+There are currently 3 timestamps in the inode that are updated to the current
+wallclock time on different activity:
+
+ctime:
+  The inode change time. This is stamped with the current time whenever
+  the inode's metadata is changed. Note that this value is not settable
+  from userland.
+
+mtime:
+  The inode modification time. This is stamped with the current time
+  any time a file's contents change.
+
+atime:
+  The inode access time. This is stamped whenever an inode's contents are
+  read. Widely considered to be a terrible mistake. Usually avoided with
+  options like noatime or relatime.
+
+Updating the mtime always implies a change to the ctime, but updating the
+atime due to a read request does not.
+
+Multigrain timestamps are only tracked for the ctime and the mtime. atimes are
+not affected and always use the coarse-grained value (subject to the floor).
+
+Inode Timestamp Ordering
+========================
+
+In addition to just providing info about changes to individual files, file
+timestamps also serve an important purpose in applications like "make". These
+programs measure timestamps in order to determine whether source files might be
+newer than cached objects.
+
+Userland applications like make can only determine ordering based on
+operational boundaries. For a syscall those are the syscall entry and exit
+points. For io_uring or nfsd operations, that's the request submission and
+response. In the case of concurrent operations, userland can make no
+determination about the order in which things will occur.
+
+For instance, if a single thread modifies one file, and then another file in
+sequence, the second file must show an equal or later mtime than the first. The
+same is true if two threads are issuing similar operations that do not overlap
+in time.
+
+If however, two threads have racing syscalls that overlap in time, then there
+is no such guarantee, and the second file may appear to have been modified
+before, after or at the same time as the first, regardless of which one was
+submitted first.
+
+Note that the above assumes that the system doesn't experience a backward jump
+of the realtime clock. If that occurs at an inopportune time, then timestamps
+can appear to go backward, even on a properly functioning system.
+
+Multigrain Timestamp Implementation
+===================================
+Multigrain timestamps are aimed at ensuring that changes to a single file are
+always recognizable, without violating the ordering guarantees when multiple
+different files are modified. This affects the mtime and the ctime, but the
+atime will always use coarse-grained timestamps.
+
+It uses an unused bit in the i_ctime_nsec field to indicate whether the mtime
+or ctime has been queried. If either or both have, then the kernel takes
+special care to ensure the next timestamp update will display a visible change.
+This ensures tight cache coherency for use-cases like NFS, without sacrificing
+the benefits of reduced metadata updates when files aren't being watched.
+
+The Ctime Floor Value
+=====================
+It's not sufficient to simply use fine or coarse-grained timestamps based on
+whether the mtime or ctime has been queried. A file could get a fine grained
+timestamp, and then a second file modified later could get a coarse-grained one
+that appears earlier than the first, which would break the kernel's timestamp
+ordering guarantees.
+
+To mitigate this problem, maintain a global floor value that ensures that
+this can't happen. The two files in the above example may appear to have been
+modified at the same time in such a case, but they will never show the reverse
+order. To avoid problems with realtime clock jumps, the floor is managed as a
+monotonic ktime_t, and the values are converted to realtime clock values as
+needed.
+
+Implementation Notes
+====================
+Multigrain timestamps are intended for use by local filesystems that get
+ctime values from the local clock. This is in contrast to network filesystems
+and the like that just mirror timestamp values from a server.
+
+For most filesystems, it's sufficient to just set the FS_MGTIME flag in the
+fstype->fs_flags in order to opt-in, providing the ctime is only ever set via
+inode_set_ctime_current(). If the filesystem has a ->getattr routine that
+doesn't call generic_fillattr, then it should call fill_mg_cmtime() to
+fill those values. For setattr, it should use setattr_copy() to update the
+timestamps, or otherwise mimic its behavior.