Re: [PATCH ] xfs: check for COW overflows in i_delayed_blks

[Eryu Guan <guaneryu@gmail.com>]

On Tue, May 28, 2019 at 10:01:32AM -0700, Darrick J. Wong wrote:
> On Sun, May 26, 2019 at 10:27:35PM +0800, Eryu Guan wrote:
> > On Mon, May 20, 2019 at 03:31:52PM -0700, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > > 
> > > With the new copy on write functionality it's possible to reserve so
> > > much COW space for a file that we end up overflowing i_delayed_blks.
> > > The only user-visible effect of this is to cause totally wrong i_blocks
> > > output in stat, so check for that.
> > > 
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > I hit xfs_db killed by OOM killer (2 vcpu, 8G memory kvm guest) when
> > trying this test and the test takes too long time (I changed the fs size
> > from 300T to 300G and tried a test run), perhaps that's why you don't
> > put it in auto group?
> 
> Oh.  Right.  I forget that I patched out xfs_db from
> check_xfs_filesystem on my dev tree years ago.
> 
> Um... do we want to remove xfs_db from the check function?  Or just open
> code a call to xfs_repair $SCRATCH_MNT/a.img at the end of the test?

If XFS maintainer removes the xfs_check call in _check_xfs_filesystem(),
I'd say I like to see it being removed :)

> 
> As for the 300T size, the reason I picked that is to force the
> filesystem to have large enough AGs to support the maximum cowextsize
> hint.  I'll see if it still works with a 4TB filesystem.

After removeing the xfs_db call, I can finish the test within 20s on the
same test vm, and a.img only takes 159MB space on $SCRATCH_DEV, so I
think 300T fs size is fine.

Thanks,
Eryu