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

[Konstantin Khlebnikov]

On 09/04/2020 13.23, Amir Goldstein wrote:
> 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

Thanks

> 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...

Now I see reason behind "sync; sync; sync; reboot" =)

Order old -> new allows to not miss new items if list modifies.
Might be important for some users.

bdev iteration seems already reversed: inode_sb_list_add adds to the head

> Thanks,
> Amir.