On Thu, Dec 02, 2021 at 02:45:24PM +0000, Filipe Manana wrote: > On Wed, Dec 01, 2021 at 01:18:52PM -0500, Josef Bacik wrote: > > We've been seeing transient errors with any test that uses a dm device > > for the entirety of the time that we've been running nightly xfstests > > I have been having it on my tests vms since ever as well. > It's really annoying, but fortunatelly it doesn't happen too often. > Yeah before this change we'd fail ~6 tests on every configruation on every overnight run. With this change we fail 0. It's rare, but with our continual testing it happens sooooooo much. > > runs. This turns out to be because sometimes we get EBUSY while trying > > to create our new dm device. Generally this is because the test comes > > right after another test that messes with the dm device, and thus we > > still have udev messing around with the device when DM tries to O_EXCL > > the block device. > > > > Add a UDEV_SETTLE_PROG before creating the device to make sure we can > > create our new dm device without getting this transient error. > > I suspect this might only make it seem the problem goes away but does not > really fix it. > > I say that for 2 reasons: > > 1) All tests that use dm end up calling _dmsetup_remove(), like through > _log_writes_remove() or _cleanup_flakey() for example. Normally those > are called in the _cleanup() function, which ensures it's done even if > the test fails for some reason. > > So I don't understand why we need that UDEV_SETTLE_PROG at _dmsetup_create(). > > And I've seen the ebusy failure happen even when the previous tests did > not use any dm device; > > 2) Some tests fail after creating the dm device and using it. For example > btrfs/206 often fails when it tries to fsck the filesystem: > > btrfs/206 3s ... [failed, exit status 1]- output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad) > --- tests/btrfs/206.out 2020-10-16 23:13:46.554152652 +0100 > +++ /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad 2021-12-01 21:09:46.317632589 +0000 > @@ -3,3 +3,5 @@ > XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) > wrote 8192/8192 bytes at offset 0 > XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec) > +_check_btrfs_filesystem: filesystem on /dev/sdc is inconsistent > +(see /home/fdmanana/git/hub/xfstests/results//btrfs/206.full for details) > ... > > (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/206.out /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad' to see the entire diff) > > In the .full file I got: > > (...) > replaying 1239@11201: sector 2173408, size 16384, flags 0x10(METADATA) > replaying 1240@11234: sector 0, size 0, flags 0x1(FLUSH) > replaying 1241@11235: sector 128, size 4096, flags 0x12(FUA|METADATA) > _check_btrfs_filesystem: filesystem on /dev/sdc is inconsistent > *** fsck.btrfs output *** > ERROR: cannot open device '/dev/sdc': Device or resource busy > ERROR: cannot open file system > Opening filesystem to check... > *** end fsck.btrfs output > *** mount output *** > > The ebusy failure is not when the test starts, but when somewhere in the middle > of the replay loop when it calls fsck, or when it ends and the fstests framework > calls fsck. > > I've seen that with btrfs/172 too, which also uses dm logwrites in a similar way. > > So to me this suggests 2 things: > > 1) Calling UDEV_SETTLE_PROG at _dmsetup_create() doesn't solve that problem with > btrfs/206 (and other tests) - the problem is fsck failing to open the scratch > device after it called _log_writes_remove() -> _dmsetup_remove(), and not a > failure to create the dm device; > > 2) The problem is likely something missing at _dmsetup_remove(). Perhaps add > another UDEV_SETTLE_PROG there: > > diff --git a/common/rc b/common/rc > index 8e351f17..22b34677 100644 > --- a/common/rc > +++ b/common/rc > @@ -4563,6 +4563,7 @@ _dmsetup_remove() > $UDEV_SETTLE_PROG >/dev/null 2>&1 > $DMSETUP_PROG remove "$@" >>$seqres.full 2>&1 > $DMSETUP_PROG mknodes >/dev/null 2>&1 > + $UDEV_SETTLE_PROG >/dev/null 2>&1 > } > > _dmsetup_create() > > I can't say if that change to _dmsetup_remove() is correct, or what it's > needed, as I really haven't spent time trying to figure out why the issue > happens. > I actually tried a few iterations before I settled on this one, but I was only trying to reproduce the EBUSY when creating the flakey device, I hadn't seen it with fsck. So I originally started with your change, but it didn't fix the problem. Then I did both, UDEV_SETTLE at the end of remove and at the beginning of create and the problem went away, and then I removed the one from remove and the problem still was gone. But since I've made this change I also have been investigating another problem where we'll get EBUSY at mkfs time when we use SCRATCH_DEV_POOL. I have a test patch in our staging branch to make sure it actuall fixes it, but I have to add this to the start of _scratch_pool_mkfs as well. It turns out that udev is doing this thing where it writes to /sys/block/whatever/uevent to make sure that a KOBJ_CHANGE event gets sent out for the device. This is racing with the test doing a mount. So the mount gets O_EXCL, which means the uevent doesn't get emitted until umount. This would explain what you're seeing, we umount, we get the KOBJ_CHANGE event once the O_EXCL is dropped, udev runs, and then fsck gets an EBUSY. This is a very long email to say that udev is causing spurious failures because of behavior I don't entirely understand. We're going to need to sprinkle in UDEV_SETTLE_PROG in different places to kill all of these different scenarios. What do we think is best here? Put UDEV_SETTLE_PROG at the start of any function that needs to do O_EXCL? So this would mean _dmsetup_create _dmsetup_remove *_mkfs *_mount *_check That would be safest, and I can certainly do that. My initial approach was just to do it where it was problematic, but the debugging I did yesterday around btrfs/223 failures and the fact that udev is queue'ing up events that get delivered at some point in the future makes it kind of hard to handle on a case-by-case basis. Thanks, Josef
