On Mon, Apr 03, 2017 at 04:00:55PM +0200, Jan Kara wrote: > On Sun 02-04-17 09:05:26, Dave Chinner wrote: > > On Thu, Mar 30, 2017 at 12:12:31PM -0400, J. Bruce Fields wrote: > > > On Thu, Mar 30, 2017 at 07:11:48AM -0400, Jeff Layton wrote: > > > > On Thu, 2017-03-30 at 08:47 +0200, Jan Kara wrote: > > > > > Because if above is acceptable we could make reported i_version to be a sum > > > > > of "superblock crash counter" and "inode i_version". We increment > > > > > "superblock crash counter" whenever we detect unclean filesystem shutdown. > > > > > That way after a crash we are guaranteed each inode will report new > > > > > i_version (the sum would probably have to look like "superblock crash > > > > > counter" * 65536 + "inode i_version" so that we avoid reusing possible > > > > > i_version numbers we gave away but did not write to disk but still...). > > > > > Thoughts? > > > > > > How hard is this for filesystems to support? Do they need an on-disk > > > format change to keep track of the crash counter? > > > > Yes. We'll need version counter in the superblock, and we'll need to > > know what the increment semantics are. > > > > The big question is how do we know there was a crash? The only thing > > a journalling filesystem knows at mount time is whether it is clean > > or requires recovery. Filesystems can require recovery for many > > reasons that don't involve a crash (e.g. root fs is never unmounted > > cleanly, so always requires recovery). Further, some filesystems may > > not even know there was a crash at mount time because their > > architecture always leaves a consistent filesystem on disk (e.g. COW > > filesystems).... > > What filesystems can or cannot easily do obviously differs. Ext4 has a > recovery flag set in superblock on RW mount/remount and cleared on > umount/RO remount. Even this doesn't help. A recent bug that was reported to the XFS list - turns out that systemd can't remount-ro the root filesystem sucessfully on shutdown because there are open write fds on the root filesystem when it attempts the remount. So it just reboots without a remount-ro. This uncovered a bug in grub in that it (still!) thinks sync(1) is sufficient to get all the metadata that points to a kernel image onto disk in places it can read. XFS, like ext4, leaves it in the journal and so the system then fails to boot because systemd didn't remount-ro the root fs and hence the journal was never flushed before reboot and so grub can't find the kernel and so everything fails.... > This flag being set on mount would imply incrementing the crash > counter. It should be pretty easy for each filesystem to implement > such flag and the counter but I agree it requires an on-disk > format change. Yup, anything we want that is persistent and consistent across filesystems will need on-disk format changes. Hence we need a solid specification first, not to mention tests to validate correct behaviour across all filesystems in xfstests... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
