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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Thu, 9 Apr 2020 13:23:29 +0300
Message-ID: <CAOQ4uxgTtbb-vDQNnY1_7EzQ=p5p2MqkfyZo2zkFQ1Wv29uqCA@mail.gmail.com>
Subject: Re: [PATCH] ovl: skip overlayfs superblocks at global sync
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>,
        Linux Containers <containers@lists.linux-foundation.org>,
        Theodore Tso <tytso@mit.edu>
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On Thu, Apr 9, 2020 at 11:30 AM Konstantin Khlebnikov
<khlebnikov@yandex-team.ru> wrote:
>
> Stacked filesystems like overlayfs has no own writeback, but they have to
> forward syncfs() requests to backend for keeping data integrity.
>
> During global sync() each overlayfs instance calls method ->sync_fs()
> for backend although it itself is in global list of superblocks too.
> As a result one syscall sync() could write one superblock several times
> and send multiple disk barriers.
>
> This patch adds flag SB_I_SKIP_SYNC into sb->sb_iflags to avoid that.
>
> Reported-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> ---

Seems reasonable.
You may add:
Reviewed-by: Amir Goldstein <amir73il@gmail.com>

+CC: containers list

This bring up old memories.
I posted this way back to fix handling of emergency_remount() in the
presence of loop mounted fs:
https://lore.kernel.org/linux-ext4/CAA2m6vfatWKS1CQFpaRbii2AXiZFvQUjVvYhGxWTSpz+2rxDyg@mail.gmail.com/

But seems to me that emergency_sync() and sync(2) are equally broken
for this use case.

I wonder if anyone cares enough about resilience of loop mounted fs to try
and change the iterate_* functions to iterate supers/bdevs in reverse order...

Thanks,
Amir.