[PATCH v4 5/7] xfs: fill dirty folios on zero range of unwritten mappings

[Brian Foster <bfoster@redhat.com>]

Use the iomap folio batch mechanism to select folios to zero on zero
range of unwritten mappings. Trim the resulting mapping if the batch
is filled (unlikely for current use cases) to distinguish between a
range to skip and one that requires another iteration due to a full
batch.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
---
 fs/xfs/xfs_iomap.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 25058004de91..521fef4d04c8 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1693,6 +1693,8 @@ xfs_buffered_write_iomap_begin(
 	struct iomap		*iomap,
 	struct iomap		*srcmap)
 {
+	struct iomap_iter	*iter = container_of(iomap, struct iomap_iter,
+						     iomap);
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
 	xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
@@ -1764,6 +1766,7 @@ xfs_buffered_write_iomap_begin(
 	 */
 	if (flags & IOMAP_ZERO) {
 		xfs_fileoff_t eof_fsb = XFS_B_TO_FSB(mp, XFS_ISIZE(ip));
+		u64 end;
 
 		if (isnullstartblock(imap.br_startblock) &&
 		    offset_fsb >= eof_fsb)
@@ -1771,6 +1774,26 @@ xfs_buffered_write_iomap_begin(
 		if (offset_fsb < eof_fsb && end_fsb > eof_fsb)
 			end_fsb = eof_fsb;
 
+		/*
+		 * Look up dirty folios for unwritten mappings within EOF.
+		 * Providing this bypasses the flush iomap uses to trigger
+		 * extent conversion when unwritten mappings have dirty
+		 * pagecache in need of zeroing.
+		 *
+		 * Trim the mapping to the end pos of the lookup, which in turn
+		 * was trimmed to the end of the batch if it became full before
+		 * the end of the mapping.
+		 */
+		if (imap.br_state == XFS_EXT_UNWRITTEN &&
+		    offset_fsb < eof_fsb) {
+			loff_t len = min(count,
+					 XFS_FSB_TO_B(mp, imap.br_blockcount));
+
+			end = iomap_fill_dirty_folios(iter, offset, len);
+			end_fsb = min_t(xfs_fileoff_t, end_fsb,
+					XFS_B_TO_FSB(mp, end));
+		}
+
 		xfs_trim_extent(&imap, offset_fsb, end_fsb - offset_fsb);
 	}
 
-- 
2.50.1