xfs: improve mapping type check in xfs_vm_writepage

Currently we only refuse a "read-only" mapping for writing out
unwritten and delayed buffers, and refuse any other for overwrites.
Improve the checks to require delalloc mappings for delayed buffers,
and unwritten extent mappings for unwritten extents.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/linux-2.6/xfs_aops.c

@@ -1082,17 +1082,17 @@ xfs_vm_writepage(
 		if (buffer_unwritten(bh) || buffer_delay(bh)) {
 			int new_ioend = 0;
 
-			/*
-			 * Make sure we don't use a read-only iomap
-			 */
-			if (flags == BMAPI_READ)
-				imap_valid = 0;
-
 			if (buffer_unwritten(bh)) {
-				type = IO_UNWRITTEN;
+				if (type != IO_UNWRITTEN) {
+					type = IO_UNWRITTEN;
+					imap_valid = 0;
+				}
 				flags = BMAPI_WRITE | BMAPI_IGNSTATE;
 			} else if (buffer_delay(bh)) {
-				type = IO_DELAY;
+				if (type != IO_DELAY) {
+					type = IO_DELAY;
+					imap_valid = 0;
+				}
 				flags = BMAPI_ALLOCATE;
 
 				if (wbc->sync_mode == WB_SYNC_NONE)
@@ -1128,8 +1128,11 @@ xfs_vm_writepage(
 			 * That means it must already have extents allocated
 			 * underneath it. Map the extent by reading it.
 			 */
-			if (!imap_valid || flags != BMAPI_READ) {
+			if (flags != BMAPI_READ) {
 				flags = BMAPI_READ;
+				imap_valid = 0;
+			}
+			if (!imap_valid) {
 				size = xfs_probe_cluster(inode, page, bh, head);
 				err = xfs_map_blocks(inode, offset, size,
 						&imap, flags);