dm: max_segments=1 if merge_bvec_fn is not supported

Commit Message

Lars Ellenberg March 6, 2010, 9:10 p.m. UTC

None

Patch

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 4b22feb..c686ff4 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -515,14 +515,22 @@  int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
 
 	/*
 	 * Check if merge fn is supported.
-	 * If not we'll force DM to use PAGE_SIZE or
+	 * If not we'll force DM to use single bio_vec of PAGE_SIZE or
 	 * smaller I/O, just to be safe.
 	 */
 
-	if (q->merge_bvec_fn && !ti->type->merge)
+	if (q->merge_bvec_fn && !ti->type->merge) {
 		limits->max_sectors =
 			min_not_zero(limits->max_sectors,
 				     (unsigned int) (PAGE_SIZE >> 9));
+		/* Restricting max_sectors is not enough.
+		 * If someone uses bio_add_page to add 8 disjunct 512 byte
+		 * partial pages to a bio, it would succeed,
+		 * but could still cross a border of whatever restrictions
+		 * are below us (e.g. raid0 stripe boundary).  An attempted
+		 * bio_split() would not succeed, because bi_vcnt is 8. */
+		limits->max_segments = 1;
+	}
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_set_device_limits);