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SubjectRe: [PATCH 2/2] Documentation/filesystems/proc.rst: document procfs inode timestamps
On Sat, Jul 23, 2022 at 12:29 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> The timestamps for procfs files are not well understood and can
> confuse users and developers [0] in particular for the timestamp
> for the start time or a process. Clarify what they mean and that
> they are a reflection of the ephemeral nature of the filesystem
> inodes.
>
> The procfs inodes are created when you first read them and then
> stuffed in the page cache. If the page cache and indodes are
> reclaimed they can be removed, and re-created with a new timestamp
> after read again. Document this little bit of tribal knowledge.
>
> [0] https://lkml.kernel.org/r/20220721081617.36103-1-zhangyuchen.lcr@bytedance.com
> Reported-by: Zhang Yuchen <zhangyuchen.lcr@bytedance.com>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
> Documentation/filesystems/proc.rst | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 9fd5249f1a5f..9defe9af683a 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -59,6 +59,15 @@ The proc file system acts as an interface to internal data structures in the
> kernel. It can be used to obtain information about the system and to change
> certain kernel parameters at runtime (sysctl).
>
> +The proc files are dynamic in nature and allow for developers to make the
> +content to be changed each time a file is read. The proc files and directories
> +inodes are created when someone first reads a respective proc file or directory,
> +as such the timestamps of the proc files reflect this time. As with other
> +filesystems, these proc inodes can be removed through reclaim under memory
> +pressure and so the timestamps of the proc files can change if the proc files
> +are destroyed and re-created (echo 3 > /proc/sys/vm/drop_caches forces and
> +illustrate the reclaim of inodes and page cache).

Thanks for fixing this.

> +
> First, we'll take a look at the read-only parts of /proc. In Chapter 2, we
> show you how you can use /proc/sys to change settings.
>
> @@ -328,6 +337,13 @@ It's slow but very precise.
> system call
> ============= ===============================================================
>
> +Note that the start_time inside the stat file is different than the timestamp
> +of the stat file itself. The timestamp of the stat file simply reflects the
> +first time the stat file was read. The proc inode for this file can be reclaimed
> +under memory pressure and be recreated after this and so the timestamp can
> +change. Userspace should rely on the start_time entry in the the stat file to
> +get a process start time.
> +

I'm not sure the value of those comments since the above description
is already enough to tell people the timestamp of /proc files or directories
can be changed in some cases, which already includes the case of
/proc/PID/stat.

If we really want to take /proc/PID/stat as an example to show the
timestamp is unstable, I think it is better to move those comments to the
above section where you explain why the timestamp can be changed .

Thanks.

> The /proc/PID/maps file contains the currently mapped memory regions and
> their access permissions.
>
> --
> 2.35.1
>

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