[PATCH 3/3] xfs: measure all contiguous previous extents for prealloc size

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

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>
---
 fs/xfs/xfs_iomap.c |   63 +++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 52 insertions(+), 11 deletions(-)


diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index ac970b13b1f8..2dffd56a433c 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -351,6 +351,46 @@ xfs_quota_calc_throttle(
 	}
 }
 
+/*
+ * Determine if the previous extent's range of offsets is contiguous with
+ * @offset_fsb.  If so, set @prev_contig to the number of blocks that are
+ * physically contiguous with that previous extent and return true.  If there
+ * is no previous extent or there's a hole right before @offset_fsb, return
+ * false.
+ *
+ * Note that we don't care if the previous extents are written or not.
+ */
+static inline bool
+xfs_iomap_prev_contiguous(
+	struct xfs_ifork	*ifp,
+	struct xfs_iext_cursor	*cur,
+	xfs_fileoff_t		offset_fsb,
+	xfs_extlen_t		*prev_contig)
+{
+	struct xfs_iext_cursor	ncur = *cur;
+	struct xfs_bmbt_irec	got, old;
+
+	xfs_iext_prev(ifp, &ncur);
+	if (!xfs_iext_get_extent(ifp, &ncur, &old))
+		return false;
+	if (old.br_startoff + old.br_blockcount < offset_fsb)
+		return false;
+
+	*prev_contig = old.br_blockcount;
+
+	xfs_iext_prev(ifp, &ncur);
+	while (xfs_iext_get_extent(ifp, &ncur, &got) &&
+	       got.br_blockcount + got.br_startoff == old.br_startoff &&
+	       got.br_blockcount + got.br_startblock == old.br_startblock &&
+	       *prev_contig <= MAXEXTLEN) {
+		*prev_contig += got.br_blockcount;
+		old = got; /* struct copy */
+		xfs_iext_prev(ifp, &ncur);
+	}
+
+	return true;
+}
+
 /*
  * If we are doing a write at the end of the file and there are no allocations
  * past this one, then extend the allocation out to the file system's write
@@ -380,12 +420,12 @@ xfs_iomap_prealloc_size(
 	struct xfs_mount	*mp = ip->i_mount;
 	struct xfs_ifork	*ifp = XFS_IFORK_PTR(ip, whichfork);
 	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
-	struct xfs_bmbt_irec	prev;
 	int			shift = 0;
 	int64_t			freesp;
 	xfs_fsblock_t		qblocks;
 	int			qshift = 0;
 	xfs_fsblock_t		alloc_blocks = 0;
+	xfs_extlen_t		plen = 0;
 
 	if (offset + count <= XFS_ISIZE(ip))
 		return 0;
@@ -400,9 +440,9 @@ xfs_iomap_prealloc_size(
 	 */
 	if ((mp->m_flags & XFS_MOUNT_ALLOCSIZE) ||
 	    XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_dalign) ||
-	    !xfs_iext_peek_prev_extent(ifp, icur, &prev) ||
-	    prev.br_startoff + prev.br_blockcount < offset_fsb)
+	    !xfs_iomap_prev_contiguous(ifp, icur, offset_fsb, &plen)) {
 		return mp->m_allocsize_blocks;
+	}
 
 	/*
 	 * Determine the initial size of the preallocation. We are beyond the
@@ -413,15 +453,16 @@ xfs_iomap_prealloc_size(
 	 * preallocation size.
 	 *
 	 * If the extent is a hole, then preallocation is essentially disabled.
-	 * Otherwise we take the size of the preceding data extent as the basis
-	 * for the preallocation size. If the size of the extent is greater than
-	 * half the maximum extent length, then use the current offset as the
-	 * basis. This ensures that for large files the preallocation size
-	 * always extends to MAXEXTLEN rather than falling short due to things
-	 * like stripe unit/width alignment of real extents.
+	 * Otherwise we take the size of the contiguous preceding data extents
+	 * as the basis for the preallocation size. If the size of the extent
+	 * is greater than half the maximum extent length, then use the current
+	 * offset as the basis. This ensures that for large files the
+	 * preallocation 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 (plen <= (MAXEXTLEN >> 1))
+		alloc_blocks = plen << 1;
 	else
 		alloc_blocks = XFS_B_TO_FSB(mp, offset);
 	if (!alloc_blocks)