[Bug 202053] [xfstests generic/464]: XFS corruption and Assertion failed: 0, file: fs/xfs/xfs_super.c, line: 985

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=202053

--- Comment #9 from Dave Chinner (david@fromorbit.com) ---
On Sun, Jan 06, 2019 at 08:31:20AM +1100, Dave Chinner wrote:
> On Fri, Jan 04, 2019 at 07:32:17AM -0500, Brian Foster wrote:
> > On Tue, Dec 25, 2018 at 06:10:59AM +0000,
> bugzilla-daemon@bugzilla.kernel.org wrote:
> > - writepages is in progress on a particular file that has decently sized
> >   post-eof speculative preallocation
> > - writepages gets to the point where it looks up or allocates a new imap
> >   that includes the preallocation, the allocation/lookup result is
> >   stored in wpc
> > - the file is closed by one process, killing off preallocation, then
> >   immediately appended to by another, updating the file size by a few
> >   bytes
> > - writepages comes back around to xfs_map_blocks() and trims imap to the
> >   current size, but imap still includes one block of the original
> speculative
> >   prealloc (that was truncated and recreated) because the size increased
> >   between the time imap was stored and trimmed
> 
> I'm betting hole punch can cause similar problems with cached maps
> in writepage. I wrote a patch yonks ago to put a generation number
> in the extent fork and to store it in the cached map, and to
> invalidate the cached map if they didn't match.
> 
> > The EOF trim approach is known to be a bandaid and potentially racy, but
> > ISTM that this problem can be trivially avoided by moving or adding
> > trims of wpc->imap immediately after a new one is cached. I don't
> > reproduce the problem so far with a couple such extra calls in place.
> > 
> > Bigger picture, we need some kind of invalidation mechanism similar to
> > what we're already doing for dealing with the COW fork in this writeback
> > context. I'm not sure the broad semantics used by the COW fork sequence
> > counter mechanism is really suitable for the data fork because any
> > extent-related change in the fork would cause an invalidation, but I am
> > wondering if we could define some subset of less frequent operations for
> > the same mechanism to reliably invalidate (e.g., on eofblocks trims, for
> > starters).
> 
> The patch I had is below - I haven't forward ported it or anything,
> just pulled it from my archive to demonstrate what I think we
> probably need to be doing here. If we want to limit when it causes
> invalidations, then we need probably need to limit which operations
> cause the generation number for that inode fork to be bumped.

Ugh. Didn't attach patch.

-Dave.

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