Re: [PATCH 4/4] xfs: split indlen reservations fairly when under reserved

[Brian Foster <bfoster@redhat.com>]

On Tue, Feb 14, 2017 at 04:12:44PM +0800, Eryu Guan wrote:
> On Thu, Feb 09, 2017 at 02:34:53PM -0500, Brian Foster wrote:
> > Certain workoads that punch holes into speculative preallocation can
> > cause delalloc indirect reservation splits when the delalloc extent is
> > split in two. If further splits occur, an already short-handed extent
> > can be split into two in a manner that leaves zero indirect blocks for
> > one of the two new extents. This occurs because the shortage is large
> > enough that the xfs_bmap_split_indlen() algorithm completely drains the
> > requested indlen of one of the extents before it honors the existing
> > reservation.
> > 
> > This ultimately results in a warning from xfs_bmap_del_extent(). This
> > has been observed during file copies of large, sparse files using 'cp
> > --sparse=always.'
> > 
> > To avoid this problem, update xfs_bmap_split_indlen() to explicitly
> > apply the reservation shortage fairly between both extents. This smooths
> > out the overall indlen shortage and defers the situation where we end up
> > with a delalloc extent with zero indlen reservation to extreme
> > circumstances.
> > 
> > Reported-by: Patrick Dung <mpatdung@gmail.com>
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > ---
> >  fs/xfs/libxfs/xfs_bmap.c | 61 ++++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 43 insertions(+), 18 deletions(-)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 49edea4..e27b9ee 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -4790,6 +4790,7 @@ xfs_bmap_split_indlen(
> >  	xfs_filblks_t			len2 = *indlen2;
> >  	xfs_filblks_t			nres = len1 + len2; /* new total res. */
> >  	xfs_filblks_t			stolen = 0;
> > +	xfs_filblks_t			resfactor;
> >  
> >  	trace_printk("%d: ores %llu len1 %llu len2 %llu\n", __LINE__, ores, len1, len2);
> 
> Is this line a leftover from debug code? Patch doesn't apply with this
> line on top of 4.10-rc8 kernel.
> 

Oops.. looks like I forgot to pull that stuff out of my branch. I'll
post a new set with it removed.

Brian

> Thanks,
> Eryu
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