Re: [PATCH v3 5/9] Documentation: add a new file documenting multigrain timestamps

[Randy Dunlap <rdunlap@infradead.org>]

On 7/5/24 10:02 AM, Jeff Layton wrote:
> Add a high-level document that describes how multigrain timestamps work,
> rationale for them, and some info about implementation and tradeoffs.
> 
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
>  Documentation/filesystems/multigrain-ts.rst | 120 ++++++++++++++++++++++++++++
>  1 file changed, 120 insertions(+)
> 
> diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst
> new file mode 100644
> index 000000000000..70d36955bb83
> --- /dev/null
> +++ b/Documentation/filesystems/multigrain-ts.rst
> @@ -0,0 +1,120 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=====================
> +Multigrain Timestamps
> +=====================
> +
> +Introduction
> +============
> +Historically, the kernel has always used a coarse time values to stamp

                                       used coarse time values

> +inodes. This value is updated on 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 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.
> +
> +Multigrain Timestamps
> +=====================
> +Multigrain timestamps are aimed at ensuring that changes to a single file are
> +always recognizeable, without violating the ordering guarantees when multiple

          recognizable
according to what I can find on the web.

> +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, we 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 you should have it call fill_mg_cmtime to
> +fill those values.
> 

-- 
~Randy