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

[Konstantin Khlebnikov]

On 09/04/2020 14.48, Amir Goldstein wrote:
> On Thu, Apr 9, 2020 at 2:28 PM Konstantin Khlebnikov
> <khlebnikov@xxxxxxxxxxxxxx> wrote:
> > 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.
>
> 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.

If fs in loop mounted with barriers then sync_fs will issue
REQ_OP_FLUSH to loop device and trigger fsync() for image file.
Sync() might write something twice but data should be safe.
Without barriers this scenario is broken for sure.

Emergency remount R/O is other thing. It really needs reverse order.

> > 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.