[PATCH 4/4] xfs: rewrite and optimize the delalloc write path

[Brian Foster]

On Fri, Aug 26, 2016 at 12:03:39PM -0400, Brian Foster wrote:
> On Fri, Aug 26, 2016 at 04:33:44PM +0200, Christoph Hellwig wrote:
> > On Thu, Aug 25, 2016 at 10:37:09AM -0400, Brian Foster wrote:
...
> > (we could potentially re-derive offset_fsb from offset if we don't
> > mind the inefficieny and recalculate maxbytes_fsb.  This already
> > assumes mp is trivially derived from ip)
> > 
> > and return
> > 
> > alloc_blocks, end_fsb
> > 
> > so the function would be quite a monster in terms of its calling
> > convention.  Additionally we'd have the related by not qute the
> > same if blocks around XFS_MOUNT_DFLT_IOSIZE and the isize split
> > over two functions, which doesn't exactly help understanding
> > the flow.
> > 
> 
> Not quite sure I follow the last bit, but I don't necessarily think the
> whole thing has to be boxed into a helper to clean it up. E.g., I'd do
> something like the appended diff (compile tested only).
> 

... and if the function signature is really an issue, trade off idx &
prev for a conditional base preallocation size (applies on top of the
previous diff):

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 45e5268..c748429 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -381,7 +381,7 @@ STATIC xfs_fsblock_t
 xfs_iomap_prealloc_size(
 	struct xfs_inode	*ip,
 	xfs_off_t		offset,
-	struct xfs_bmbt_irec	*prev)
+	xfs_extlen_t		base)
 {
 	struct xfs_mount	*mp = ip->i_mount;
 	int			shift = 0;
@@ -406,8 +406,8 @@ xfs_iomap_prealloc_size(
 	 * always extends to MAXEXTLEN rather than falling short due to things
 	 * like stripe unit/width alignment of real extents.
 	 */
-	if (prev->br_blockcount <= (MAXEXTLEN >> 1))
-		alloc_blocks = prev->br_blockcount << 1;
+	if (base <= (MAXEXTLEN >> 1))
+		alloc_blocks = base << 1;
 	else
 		alloc_blocks = XFS_B_TO_FSB(mp, offset);
 	if (!alloc_blocks)
@@ -506,12 +506,10 @@ xfs_iomap_prealloc(
 	struct xfs_inode	*ip,
 	loff_t			offset,
 	loff_t			count,
-	xfs_extnum_t		idx,
-	struct xfs_bmbt_irec	*prev)
+	xfs_extlen_t		base)
 {
 	struct xfs_mount	*mp = ip->i_mount;
 	xfs_fsblock_t		alloc_blocks;
-	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
 
 	if (offset + count <= XFS_ISIZE(ip))
 		return 0;
@@ -526,12 +524,11 @@ xfs_iomap_prealloc(
 	 */
 	if ((mp->m_flags & XFS_MOUNT_DFLT_IOSIZE) ||
 	    XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign) ||
-	    idx == 0 ||
-	    prev->br_startoff + prev->br_blockcount < offset_fsb)
+	    !base)
 		alloc_blocks = mp->m_writeio_blocks;
 	else
 		alloc_blocks =
-			xfs_iomap_prealloc_size(ip, offset, prev);
+			xfs_iomap_prealloc_size(ip, offset, base);
 
 	return alloc_blocks;
 }
@@ -603,8 +600,15 @@ xfs_file_iomap_begin_delay(
 	end_fsb = orig_end_fsb =
 		min(XFS_B_TO_FSB(mp, offset + count), maxbytes_fsb);
 
-	if (eof)
-		prealloc = xfs_iomap_prealloc(ip, offset, count, idx, &prev);
+	if (eof) {
+		xfs_extlen_t	base = 0;
+
+		/* use prev extent as base if contiguous */
+		if (idx > 0 &&
+		    prev.br_startoff + prev.br_blockcount < offset_fsb)
+			base = prev.br_blockcount;
+		prealloc = xfs_iomap_prealloc(ip, offset, count, base);
+	}
 	if (prealloc) {
 		xfs_off_t	aligned_offset;
 		xfs_extlen_t	align;