Re: XFS crash consistency bug : Loss of fsynced metadata operation

Dave Chinner
2018-03-14T13:33:01Z

On Wed, Mar 14, 2018 at 02:16:59PM +0100, Lukas Czerner wrote:
> On Wed, Mar 14, 2018 at 09:45:22AM +1100, Dave Chinner wrote:
> > On Tue, Mar 13, 2018 at 11:57:21AM -0500, Jayashree Mohan wrote:
> > > Hi Dave,
> > >
> > > Thanks for the response. CrashMonkey assumes the following behavior of
> > > disk cache. Let me know if any of this sounds unreasonable.
> > >
> > > Whenever the underlying storage device has an associated cache, the IO
> > > is marked completed the moment it reaches the disk cache. This does
> > > not guarantee that the disk cache would persist them in the same
> > > order, unless there is a Flush/FUA. The order of completed writes as
> > > seen by the user could be A, B, C, *Flush* D, E. However the disk
> > > cache could write these back to the persistent storage in the order
> > > say B, A, C, E, D. The only invariant it ensures is that writing in an
> > > order like A, C, E, B, D is
> > > not possible because, writes A,B,C have to strictly happen before D
> > > and E. However you cannot ensure that (A, B, C) is written to the
> > > persistent storage in the same order.
> > >
> > > CrashMonkey reorders bios in conformance to the guarantees provided by
> > > disk cache; we do not make any extra assumptions and we respect the
> > > barrier operations.
> >
> > I think your model is wrong. caches do not randomly re-order
> > completed IO operations to the *same LBA*. When a block is overwritten
> > the cache contains the overwrite data and the previous data is
> > discarded. THe previous data may be on disk, but it's no longer in
> > the cache.
> >
> > e.g. take a dependent filesystem read-modify-write cycle (I'm
> > choosing this because that's the problem this fzero/fsync
> > "bug" is apparently demonstrating) where we write data to disk,
> > invalidate the kernel cache, read the data back off disk, zero it
> > in memory, then write it back to disk, all in the one LBA:
> >
> > <flush>
> > write A to disk, invalidate kernel cache
> > ......
> > read A from disk into kernel cache
> > A' = <modify A>
> > write A' to disk
> > ......
> > <flush>
> >
> > The disk cache model you are describing allows writes
> > to be reordered anywhere in the flush window regardless of their
> > inter-IO completion dependencies. Hence you're allowing temporally
> > ordered filesystem IO to the same LBA be reorded like so:
> >
> >
> > <flush>
> > ......
> > write A'
> > ......
> > read A
> > A' = <modify A>
> > ......
> > write A
> > ......
> > <flush>
> >
> > This violates causality. it's simply *not possible for the disk
> > cache to contain A' before either "write A", "read A" or the
> > in-memory modification of A has been completed by the OS. Hence
> > there is no way for a crash situation to have the disk cache or the
> > physical storage medium to contain corruption that indicates it
> > stored A' on disk before stored A.
> >
> > > CrashMonkey therefore respects the guarantees provided by the disk
> > > cache, and assumes nothing more than that. I hope this provides more
> > > clarity on what
> > > CrashMonkey is trying to do, and why we think it is reasonable to do so.
> >
> > It clearly demonstrates to me where CrashMonkey is broken and needs
> > fixing - it needs to respect the ordering of temporally separate IO
> > to the same LBA and not violate causality. Simulators that assume
> > time travel is possible are not useful to us.
> >
> > Cheers,
>
> Hi Dave,
>
> just FYI the 042 xfstest does fail on xfs with what I think is stale
> data exposure. It might not be related at all to what crashmonkey is
> reporting but there is something wrong nevertheless.

generic/042 is unreliable and certain operations result in a
non-zero length file because of metadata commits/writeback that
occur as a result of the fallocate operations. It got removed from
the auto group because it isn't a reliable test about 3 years ago:

commit 7721b85016081cf6651d139e8af26267eceb23d3
Author: Jeff Moyer <jmoyer@redhat.com>
Date: Mon Dec 21 18:01:47 2015 +1100

generic/042: remove from the 'auto' group

This test fails 100% of the time for me on xfs and current git head, and
is not run for ext4 since ext4 does not support shutdown. After talking
with bfoster, it isn't expected to succeed right now. Since the auto
group is for tests that *are* expected to succeed, let's move this one
out.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

This is what it used to be testing - it was an XFS only test:

commit cf1438248c0f62f3b64013d23e9e6d6bc23ca24b
Author: Brian Foster <bfoster@redhat.com>
Date: Tue Oct 14 22:59:39 2014 +1100

xfs/053: test for stale data exposure via falloc/writeback interaction

XFS buffered I/O writeback has a subtle race condition that leads to
stale data exposure if the filesystem happens to crash after delayed
allocation blocks are converted on disk and before data is written back
to said blocks.

Use file allocation commands to attempt to reproduce a related, but
slightly different variant of this problem. The associated falloc
commands can lead to partial writeback that converts an extent larger
than the range affected by falloc. If the filesystem crashes after the
extent conversion but before all other cached data is written to the
extent, stale data can be exposed.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

So, generic/042 was written to expose unwritten extent conversion
issues in the XFS writeback path and has nothing to do with mutliple
writes being reordered. It's failure detection is unreliable and not
indicative of an actual failure or data corruption, and that's why
it was removed from the auto group....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com