Re: xfstests failures with xfs, dax and v4.4-rc3

[Dave Chinner <david@fromorbit.com>]

On Thu, Dec 10, 2015 at 09:54:58AM -0700, Ross Zwisler wrote:
> On Wed, Dec 02, 2015 at 02:39:32PM -0700, Ross Zwisler wrote:
> > On Thu, Dec 03, 2015 at 07:45:02AM +1100, Dave Chinner wrote:
> > > On Thu, Dec 03, 2015 at 07:29:10AM +1100, Dave Chinner wrote:
> > > > On Wed, Dec 02, 2015 at 11:34:38AM -0700, Ross Zwisler wrote:
> > > > > I'm hitting a few more test failures in my testing setup with v4.4-rc3, xfs
> > > > > and DAX.  My test setup is a pair of 4GiB PMEM partitions in a KVM virtual
> > > > > machine.  Here are the failures:
> > > > 
> > > > Which are caused by commit 1ca1915 ("xfs: Don't use unwritten extents
> > > > for DAX") because of this code for unwritten extent conversion in
> > > > get_blocks:
> > > > 
> > > > 	tp->t_flags |= XFS_TRANS_RESERVE;
> > > > 
> > > > It's a minor problem compared to all the other issues DAX has right
> > > > now, so I ignored it to get the bigger problem solved first.
> > > 
> > > Patch to fix the problem below.
> > > 
> > > -Dave.
> > > -- 
> > > Dave Chinner
> > > david@fromorbit.com
> > > 
> > > xfs: Don't use reserved blocks for data blocks with DAX
> > > 
> > > From: Dave Chinner <dchinner@redhat.com>
> > > 
> > > Commit 1ca1915 ("xfs: Don't use unwritten extents for DAX") enabled
> > > the DAX allocation call to dip into the reserve pool in case it was
> > > converting unwritten extents rather than allocating blocks. This was
> > > a direct copy of the unwritten extent conversion code, but had an
> > > unintended side effect of allowing normal data block allocation to
> > > use the reserve pool. Hence normal block allocation could deplete
> > > the reserve pool and prevent unwritten extent conversion at ENOSPC,
> > > hence violating fallocate guarantees on preallocated space.
> > > 
> > > Fix it by checking whether the incoming map from __xfs_get_blocks()
> > > spans an unwritten extent and only use the reserve pool if the
> > > allocation covers an unwritten extent.
> > > 
> > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > 
> > Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > 
> > I've verified that this fixes all three failing xfstests reported in this mail.
> > Thanks!
> 
> Hey Dave,
> 
> Are you planning on pushing this fix for v4.4?

No plans to right now - ENOSPC is a corner case that most users
won't be anywhere near, especially for experimental functionality on
hardware nobody actually has....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com