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Date: Tue, 27 Mar 2018 16:15:29 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: jiangyiwen <jiangyiwen@huawei.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>,
        Ron Minnich <rminnich@sandia.gov>, Latchesar Ionkov <lucho@ionkov.net>,
        <linux-kernel@vger.kernel.org>, <v9fs-developer@lists.sourceforge.net>,
        Greg Kurz <groug@kaod.org>
Subject: Re: [V9fs-developer] [PATCH] fs/9p: don't set SB_NOATIME by default
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On Tue, 27 Mar 2018 09:50:47 +0800 jiangyiwen <jiangyiwen@huawei.com> wrote:

> User use some syscall, for example mmap(v9fs_file_mmap), it will not
> update atime even if user's mnt_flags without MNT_NOATIME, because
> v9fs default set SB_NOATIME in v9fs_set_super.
> 
> For supporting access time is updated when user mount with relatime,
> we should not set SB_NOATIME by default.
> 
> ...
>
> --- a/fs/9p/vfs_super.c
> +++ b/fs/9p/vfs_super.c
> @@ -94,7 +94,7 @@ static int v9fs_set_super(struct super_block *s, void *data)
>  	if (v9ses->cache)
>  		sb->s_bdi->ra_pages = (VM_MAX_READAHEAD * 1024)/PAGE_SIZE;
> 
> -	sb->s_flags |= SB_ACTIVE | SB_DIRSYNC | SB_NOATIME;
> +	sb->s_flags |= SB_ACTIVE | SB_DIRSYNC;
>  	if (!v9ses->cache)
>  		sb->s_flags |= SB_SYNCHRONOUS;
> 

So strictly speaking, this is a non-backward-compatible change, yes?

Please describe the circumstances under which an existing user might be
harmed by this.  I *think* such harm will occur if the user was already
using 'mount -o relatime', yes?  They previously weren't getting
relatime treatment, but now they will, and things will be a little slower.

If correct, that sounds acceptable.