Date:	Thu, 10 Jun 2010 17:19:49 +1000
From:	Dave Chinner <david@...morbit.com>
To:	linux-fsdevel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, viro@...IV.linux.org.uk,
	josef@...hat.com, jeffmerkey@...il.com
Subject: [RFC, PATCH 0/5] fsfreeze: fix sb vs bdev freeze/thaw b0rkage

The following series is for to address bugs in the emergency thawing code, as
well as mismatcheѕ with freezing at the block layer and the superblock that
break the freeze/thaw nesting order.

The first two patches fix the emergency thaw infinite loop reported by Jeff
Merkey and the deadlock on sb->s_umount that the infinite loop hid. These may
be stable kernel candidates.

The remainder of the patches address the bdev/sb mismatch and the fact that sb
level freezing does not nest correctly. For all the places that the bdev
interfaces are used, we need a superblock anyway so we may as well make
freeze/thaw work only at the sb level. As such, this series moves all the
nesting code to the sb from the bdev level and removes the
freeze_bdev/thaw_bdev interfaces completely. It also converts the emergency
thaw to work at the superblock level such that it will now thaw manually frozen
filesystems.

A *big* outstanding problem still exists - freezing takes an active reference
to the superblock, so unmounting an frozen filesystem has some nasty and
unexpected side effects. The existing code results in an unmountable block
device:

# mount /dev/vda /mnt/test
# xfs_freeze -f /mnt/test
# umount /mnt/test
# grep test /proc/mounts
# mkfs.xfs -f -l size=128m /dev/vda
mkfs.xfs: /dev/vda contains a mounted filesystem
Usage: mkfs.xfs
....
# mount /dev/vda /mnt/test
mount: /dev/vda already mounted or /mnt/test busy
#

At this point I can't get access to /dev/vda and needs a reboot to
get it and /mnt/test back.

This patch series results in the block device being mountable, but
remains frozen across unmount/mount:

# mount /dev/vda /mnt/test
# xfs_freeze -f /mnt/test
# umount /mnt/test
# grep test /proc/mounts
# mkfs.xfs -f -l size=128m /dev/vda
mkfs.xfs: /dev/vda contains a mounted filesystem
Usage: mkfs.xfs
....
# mount /dev/vda /mnt/test
# touch /mnt/test/foo &
[1] 2647
#
# xfs_freeze -u /mnt/test
[1]+  Done                    sudo touch /mnt/test/foo
# umount /mnt/test
# mkfs.xfs -f -l size=128m /dev/vda
meta-data=/dev/vda               isize=256    agcount=4, agsize=262144 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=1048576, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=32768, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
#

This behaviour is only marginally better than the existing behaviour
(at least you can release the references). However, I don't really
like either option - we used to disallow umount on a frozen
filesystems to avoid this problem.

So What is really supposed to happen when we unmount a frozen
superblock? Should unmount return EBUSY?  Should it be automatically
thawed so it doesn't affect block device behaviour after unmount?
Something else?

Cheers,

Dave.

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