On Thu, Jun 10, 2010 at 05:19:49PM +1000, Dave Chinner wrote: > 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? > My vote is for EBUSY, that way we don't get this weird unexpected behavior thing. Plus if dm is doing a snapshot or whatever, unfreezing the fs to let it unmount in the middle of it doing its thing could be unfun. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
