On Fri, Jun 28, 2019 at 07:51:43AM +0200, Christoph Hellwig wrote: > On Thu, Jun 27, 2019 at 11:23:09AM -0700, Darrick J. Wong wrote: > > On Thu, Jun 27, 2019 at 12:48:30PM +0200, Christoph Hellwig wrote: > > > There is no real problem merging ioends that go beyond i_size into an > > > ioend that doesn't. We just need to move the append transaction to the > > > base ioend. Also use the opportunity to use a real error code instead > > > of the magic 1 to cancel the transactions, and write a comment > > > explaining the scheme. > > > > > > Signed-off-by: Christoph Hellwig <h...@lst.de> > > > > Reading through this patch, I have a feeling it fixes the crash that > > Zorro has been seeing occasionally with generic/475... > > So you think for some reason the disk i_size changes underneath and thus > the xfs_ioend_is_append misfired vs the actual transaction allocations? > I didn't even think of that, but using the different checks sure sounds > dangerous. So yes, we'd either need to backport my patch, or at least > replace the checks in xfs_ioend_can_merge with direct checks of > io_append_trans.
That's my working theory, yes. 1. Dirty pages 0 and 2 of an empty file. 2. Writeback gets scheduled for pages 0 and 2, creating ioends A and C. Both ioends describe writes past the on-disk isize so we allocate transactions. 3. ioend C completes immediately, sets the ondisk isize to (3 * PAGESIZE). 4. Dirty page 1 of the file and immediately schedule writeback for it, creating ioend B. ioend B describes a write within the on-disk isize so we do not allocate setfilesize transaction. 5. ioend A and B complete and are sorted into the per-inode ioend completion list. xfs_ioend_try_merge looks at ioend A, sees that ioend can be merged with ioend B (same type, same cow status, same current setfilesize status (which does not reflect the setfilesize status when A was created)) and therefore decides to merge them. 6. A has a setfilesize transaction so _try_merge calls xfs_setfilesize_ioend(ioend B, -1) to cancel ioend B's transaction, but as we saw in (4), ioend B has no transaction and crashes. --D
