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

[Christoph Hellwig <hch@infradead.org>]

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.