Re: [PATCH] xfs: recheck reflink / dirty page status before freeing CoW reservations

[Eryu Guan <eguan@redhat.com>]

On Wed, Jan 10, 2018 at 02:03:36PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Eryu Guan reported seeing occasional hangs when running generic/269 with
> a new fsstress that supports clonerange/deduperange.  The cause of this
> hang is an infinite loop when we convert the CoW fork extents from
> unwritten to real just prior to writing the pages out; the infinite
> loop happens because there's nothing in the CoW fork to convert, and so
> it spins forever.
> 
> The underlying issue here is that when we go to perform these CoW fork
> conversions, we're supposed to have an extent waiting for us, but the
> low space CoW reaper has snuck in and blown them away!  There are four
> conditions that can dissuade the reaper from touching our file -- no
> reflink iflag; dirty page cache; writeback in progress; or directio in
> progress.  We check the four conditions prior to taking the locks, but
> we neglect to recheck them once we have the locks, which is how we end
> up whacking the writeback that's in progress.
> 
> Therefore, refactor the four checks into a helper function and call it
> once again once we have the locks to make sure we really want to reap
> the inode.  While we're at it, add an ASSERT for this weird condition so
> that we'll fail noisily if we ever screw this up again.
> 
> Reported-by: Eryu Guan <eguan@redhat.com>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

I applied this patch on top of v4.15-rc5 kernel, and ran generic/083
generic/269 and generic/270 (where I hit the soft lockup and hang before)
multiple times and tests all passed. I also ran all tests in 'enospc'
group on 1k/2k/4k XFS with reflink enabled, tests passed too. So

Tested-by: Eryu Guan <eguan@redhat.com>

Thanks!

Eryu