Re: [PATCH] generic: skip dm-log-writes tests on XFS v5 superblock filesystems

[Dave Chinner <david@fromorbit.com>]

On Tue, Feb 26, 2019 at 11:10:02PM +0200, Amir Goldstein wrote:
> On Tue, Feb 26, 2019 at 8:14 PM Brian Foster <bfoster@redhat.com> wrote:
> >
> > The dm-log-writes mechanism runs a workload against a filesystem,
> > tracks underlying FUAs and restores the filesystem to various points
> > in time based on FUA marks. This allows fstests to check fs
> > consistency at various points and verify log recovery works as
> > expected.
> >
> 
> Inaccurate. generic/482 restores to FUA points.
> generic/45[57] restore to user defined points in time (marks).
> dm-log-writes mechanism is capable of restoring either.
> 
> > This mechanism does not play well with LSN based log recovery
> > ordering behavior on XFS v5 superblocks, however. For example,
> > generic/482 can reproduce false positive corruptions based on extent
> > to btree conversion of an inode if the inode and associated btree
> > block are written back after different checkpoints. Even though both
> > items are logged correctly in the extent-to-btree transaction, the
> > btree block can be relogged (multiple times) and only written back
> > once when the filesystem unmounts. If the inode was written back
> > after the initial conversion, recovery points between that mark and
> > when the btree block is ultimately written back will show corruption
> > because log recovery sees that the destination buffer is newer than
> > the recovered buffer and intentionally skips the buffer. This is a
> > false positive because the destination buffer was resiliently
> > written back after being physically relogged one or more times.
> >
> 
> This story doesn't add up.
> Either dm-log-writes emulated power failure correctly or it doesn't.
> My understanding is that the issue you are seeing is a result of
> XFS seeing "data from the future" after a restore of a power failure
> snapshot, because the scratch device is not a clean slate.
> If I am right, then the correct solution is to wipe the journal before
> starting to replay restore points.

If that is the problem, then I think we should be wiping the entire
block device before replaying the recorded logwrite.

i.e. this sounds like a "block device we are replaying onto has
stale data in it" problem because we are replaying the same
filesystem over the top of itself.  Hence there are no unique
identifiers in the metadata that can detect stale metadata in
the block device.

I'm surprised that we haven't tripped over this much earlier that
this...

> > Update the dm-log-writes require checks to enforce v4 superblocks
> > when running against XFS and skip the test otherwise.
> 
> You might as well disable dm-log-writes test for XFS completely.
> Who cares about v4 superblocks these days?

Enough of the inflammatory hyperbole, Amir. Statements like this
serve no useful purpose.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com