Re: [PATCH] ovl: skip overlayfs superblocks at global sync

[Amir Goldstein]

On Thu, Apr 9, 2020 at 11:30 AM Konstantin Khlebnikov
<khlebnikov@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> ---

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

+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@xxxxxxxxxxxxxx/

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.