linux-kernel - Re: [PATCH] fs, pseudo: Do not update atime for pseudo inodes

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Message-ID: <CAOQ4uxjdTUnA2ACQtyZ95QkTtH_zaKZEYLyok73yjrhuUyXmtg@mail.gmail.com>
Date:   Fri, 19 Jun 2020 16:45:09 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, pseudo: Do not update atime for pseudo inodes

On Wed, Jun 17, 2020 at 5:53 PM Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> The kernel uses internal mounts created by kern_mount() and populated
> with files with no lookup path by alloc_file_pseudo() for a variety of
> reasons. An relevant example is anonymous pipes because every vfs_write
> also checks if atime needs to be updated even though it is unnecessary.
> Most of the relevant users for alloc_file_pseudo() either have no statfs
> helper or use simple_statfs which does not return st_atime. The closest

st_atime is returned by simple_getattr()

> proxy measure is the proc fd representations of such inodes which do not
> appear to change once they are created. This patch sets the S_NOATIME
> on inode->i_flags for inodes created by new_inode_pseudo() so that atime
> will not be updated.
>

new_inode() calls new_inode_pseudo() ...
You need to factor out a new helper.

Either you can provide callers analysis of all new_inode_pseudo() users
or use a new helper to set S_NOATIME and call it from the relevant users
(pipe, socket).

How about S_NOCMTIME while you are at it?
Doesn't file_update_time() show in profiling?
Is there a valid use case for updating c/mtime of anonymous socket/pipe?

Thanks,
Amir.