Re: [BUG] xfs/109 crashed 2k block size reflink enabled XFS

[Christoph Hellwig <hch@infradead.org>]

On Mon, Dec 05, 2016 at 10:28:02AM -0800, Darrick J. Wong wrote:
> Hmm.  Purely speculating here (I haven't yet been able to reproduce it)
> but I wonder if we're nearly out of space, fdblocks is still large
> enough that we can start delalloc reservations, but something is
> stealing blocks out of the AGs such that when we go to look for one
> there aren't any (or the per AG reservation denies it).
> 
> Does it happen if rmapbt=0 ?  Since xfs/109 isn't doing any CoW, it's
> possible that this could be another symptom of the bug where we reserve
> all the bmap+rmap blocks we need via indlen, but discard the entire
> reservation in the transaction roll that happens before we start the
> rmap update, which effectively means we're allocating space that we
> didn't previously reserve...

I'm pretty sure it's something like that, but I don't think rmap
needs to be in the game - at least my customer report does not have rmap
enabled.

> I suppose you could constrict the reflink exception thing further by
> passing bma->flags to xfs_bmap_extents_to_btree and only allowing the
> ENOSPC retry if XFS_BMAPI_REMAP is set.

Well, we assert on having an allocation even without that exception,
so I'm not sure that would help us - it's just an oddity I noticed
while looking at the code.