Re: LMDB mdb_copy produces a corrupt database on btrfs, but not on ext4

[Boris Burkov <boris@bur.io>]

On Thu, Feb 16, 2023 at 09:43:03PM +0000, Filipe Manana wrote:
> On Thu, Feb 16, 2023 at 6:49 PM Christoph Hellwig <hch@infradead.org> wrote:
> >
> > On Thu, Feb 16, 2023 at 06:00:08PM +0000, Filipe Manana wrote:
> > > Ok, so the problem is btrfs_dio_iomap_end() detects the submitted
> > > amount is less than expected, so it marks the ordered extents as not
> > > up to date, setting the BTRFS_ORDERED_IOERR bit on it.
> > > That results in having an unexpected hole for the range [8192, 65535],
> > > and no error returned to btrfs_direct_write().
> > >
> > > My initial thought was to truncate the ordered extent at
> > > btrfs_dio_iomap_end(), similar to what we do at
> > > btrfs_invalidate_folio().
> > > I think that should work, however we would end up with a bookend
> > > extent (but so does your proposed fix), but I don't see an easy way to
> > > get around that.
> >
> > Wouldn't a better way to handle this be to cache the ordered_extent in
> > the btrfs_dio_data, and just reuse it on the next iteration if present
> > and covering the range?
> 
> That may work too, yes.

Quick update, I just got a preliminary version of this proposal working:
- reuse btrfs_dio_data across calls to __iomap_dio_rw
- store the dio ordered_extent when we create it in btrfs_dio_iomap_begin
- modify btrfs_dio_iomap_end to not mark the unfinished ios done in the
  incomplete case. (and to drop the ordered extent on done or error)
- modify btrfs_dio_iomap_begin to short-circuit when it has a cached
  ordered_extent

The resulting behavior on this workload is:
- write 8192
- finish OE, write file extent
- write 57344 (no extent, cached OE)
- re-enter __iomap_dio_rw with a live OE
- skip locking extent, reserving space, etc.
- write 1769472
- finish OE, write file extent

and the file looks as if there were no partial write. I think this is a
good structure for a fix to this bug, and plan to polish it up and send
it soon, unless someone objects and thinks we should go a different way.