Re: [BUG REPORT] next-20231102: generic/311 fails on XFS with external log

[Dave Chinner <david@fromorbit.com>]

On Thu, Nov 02, 2023 at 03:54:48PM +0100, Christian Brauner wrote:
> On Thu, Nov 02, 2023 at 06:06:10PM +0530, Chandan Babu R wrote:
> > Hi,
> > 
> > generic/311 consistently fails when executing on a kernel built from
> > next-20231102.
> > 
> > The following is the fstests config file that was used during testing.
> > 
> > export FSTYP=xfs
> > 
> > export TEST_DEV=/dev/loop0
> > export TEST_DIR=/mnt/test
> > export TEST_LOGDEV=/dev/loop2
> > 
> > export SCRATCH_DEV=/dev/loop1
> > export SCRATCH_MNT=/mnt/scratch
> > export SCRATCH_LOGDEV=/dev/loop3
> 
> Thanks for the report. So dm flakey sets up:
> 
> /dev/dm-0 over /dev/loop0
> /dev/dm-1 over /dev/loop2
> 
> and then we mount an xfs filesystem with:
> 
> /dev/loop2 as logdev and /dev/loop0 as the main device.
> 
> So on current kernels what happens is that if you freeze the main
> device you end up:
> 
> bdev_freeze(dm-0)
> -> get_super(dm-0) # finds xfs sb
>    -> freeze_super(sb)
> 
> if you also freeze the log device afterwards via:
> 
> bdev_freeze(dm-1)
> -> get_super(dm-1) # doesn't find xfs sb because freezing only works for
>                    # main device
> 
> What's currently in -next allows you to roughly do the following:
> 
> bdev_freeze(dm-0)
> -> fs_bdev_freeze(dm-0->sb)
>    -> freeze_super(dm-0->sb) # returns 0
> 
> bdev_freeze(dm-1)
> -> fs_bdev_freeze(dm-1->sb)
>    -> freeze_super(dm-1->sb) # returns -EBUSY
> 
> So you'll see EBUSY because the superblock was already frozen when the
> main block device was frozen. I was somewhat expecting that we may run
> into such issues.
> 
> I think we just need to figure out what we want to do in cases the
> superblock is frozen via multiple devices. It would probably be correct
> to keep it frozen as long as any of the devices is frozen?

So this series removed the blockdev freeze nesting code that dm
suspend/resume functionality used (i.e. it allowed concurrent bdev
freeze/thaw works and leaves the bdev frozen until the last thaw
occurs). Removing bd_fsfreeze_count essentially removed this nesting
ability.

IMO, bdev_freeze() should still nest freeze/thaw across all devices
in the filesystem like it used to on the main device. This implies
that freeze_super() needs to know that it is being called from
bdev_freeze() and needs a freeze counter to allow concurrent bdev
freezes and only thaw the fs when the last freeze goes away....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com