[PATCH] ovl: skip overlayfs superblocks at global sync

[Amir Goldstein]

On Thu, Apr 9, 2020 at 2:28 PM Konstantin Khlebnikov
<khlebnikov at yandex-team.ru> wrote:
>
> On 09/04/2020 13.23, Amir Goldstein wrote:
> > On Thu, Apr 9, 2020 at 11:30 AM Konstantin Khlebnikov
> > <khlebnikov at 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 at yandex-team.ru>
> >> Signed-off-by: Konstantin Khlebnikov <khlebnikov at yandex-team.ru>
> >> ---
> >
> > Seems reasonable.
> > You may add:
> > Reviewed-by: Amir Goldstein <amir73il at gmail.com>
> >
> > +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@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...
>
> 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.
>

That's not the reason I suggested reverse order.
The reason is that with loop mounted fs, the correct order of flushing is:
1. sync loop mounted fs inodes => writes to loop image file
2. sync loop mounted fs sb => fsyncs the loop image file
3. sync the loop image host fs sb

With forward sb iteration order, #3 happens before #1, so the
loop mounted fs changes are not really being made durable by
a single sync(2) call.

> bdev iteration seems already reversed: inode_sb_list_add adds to the head
>

I think bdev iteration order will not make a difference in this case.
flushing /dev/loopX will not be needed and it happens too late
anyway.

Thanks,
Amir.