Re: [PATCH 2/4] xfs: measure all contiguous previous extents for prealloc size

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, May 21, 2020 at 11:56:50PM -0700, Christoph Hellwig wrote:
> On Thu, May 21, 2020 at 07:53:09PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > When we're estimating a new speculative preallocation length for an
> > extending write, we should walk backwards through the extent list to
> > determine the number of number of blocks that are physically and
> > logically contiguous with the write offset, and use that as an input to
> > the preallocation size computation.
> > 
> > This way, preallocation length is truly measured by the effectiveness of
> > the allocator in giving us contiguous allocations without being
> > influenced by the state of a given extent.  This fixes both the problem
> > where ZERO_RANGE within an EOF can reduce preallocation, and prevents
> > the unnecessary shrinkage of preallocation when delalloc extents are
> > turned into unwritten extents.
> > 
> > This was found as a regression in xfs/014 after changing delalloc writes
> > to create unwritten extents during writeback.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Looks good,
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 
> A minor nitpick, though:
> 
> > +	struct xfs_iext_cursor	ncur = *icur; /* struct copy */
> >
> > +	struct xfs_bmbt_irec	prev, got;
> 
> The comment is pretty pointless, as the struct copy is obviously form
> the syntax (and we do it for the xfs_iext_cursor structure in quite a
> few other places).
> 
> Also please don't add empty lines between the variable declarations.

I didn't... not sure where that came from.

--D