[PATCH 3/3] xfs: report the correct read/write dio alignment for reflinked inodes

[Christoph Hellwig <hch@lst.de>]

For I/O to reflinked blocks we always need to write an entire new
file system block, and the code enforces the file system block alignment
for the entire file if it has any reflinked blocks.

Use the new STATX_DIO_READ_ALIGN flag to report the asymmetric read
vs write alignments for reflinked files.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_iops.c | 37 +++++++++++++++++++++++++++++--------
 1 file changed, 29 insertions(+), 8 deletions(-)

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 1cdc8034f54d93..de2fc12688dc23 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -570,6 +570,33 @@ xfs_stat_blksize(
 	return PAGE_SIZE;
 }
 
+static void
+xfs_report_dioalign(
+	struct xfs_inode	*ip,
+	struct kstat		*stat)
+{
+	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
+	struct block_device	*bdev = target->bt_bdev;
+
+	stat->result_mask |= STATX_DIOALIGN | STATX_DIO_READ_ALIGN;
+	stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
+	stat->dio_read_offset_align = bdev_logical_block_size(bdev);
+
+	/*
+	 * On COW inodes we are forced to always rewrite an entire file system
+	 * block.
+	 *
+	 * Because applications assume they can do sector sized direct writes
+	 * on XFS we provide an emulation by doing a read-modify-write cycle
+	 * through the cache, but that is highly inefficient.  Thus report the
+	 * natively supported size here.
+	 */
+	if (xfs_is_cow_inode(ip))
+		stat->dio_offset_align = ip->i_mount->m_sb.sb_blocksize;
+	else
+		stat->dio_offset_align = stat->dio_read_offset_align;
+}
+
 STATIC int
 xfs_vn_getattr(
 	struct mnt_idmap	*idmap,
@@ -635,14 +662,8 @@ xfs_vn_getattr(
 		stat->rdev = inode->i_rdev;
 		break;
 	case S_IFREG:
-		if (request_mask & STATX_DIOALIGN) {
-			struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
-			struct block_device	*bdev = target->bt_bdev;
-
-			stat->result_mask |= STATX_DIOALIGN;
-			stat->dio_mem_align = bdev_dma_alignment(bdev) + 1;
-			stat->dio_offset_align = bdev_logical_block_size(bdev);
-		}
+		if (request_mask & (STATX_DIOALIGN | STATX_DIO_READ_ALIGN))
+			xfs_report_dioalign(ip, stat);
 		fallthrough;
 	default:
 		stat->blksize = xfs_stat_blksize(ip);
-- 
2.43.0