[PATCH] dm: max_segments=1 if merge_bvec_fn is not supported

Lars Ellenberg
2010-03-06T21:10:17Z

If the lower device exposes a merge_bvec_fn,
dm_set_device_limits() restricts max_sectors
to PAGE_SIZE "just to be safe".

This is not sufficient, however.

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. raid10 stripe boundary).
An attempted bio_split() would not succeed, because bi_vcnt is 8.

One example that triggered this frequently is the xen io layer.

raid10_make_request bug: can't convert block across chunks or bigger than 64k 209265151 1

Signed-off-by: Lars <lars.ellenberg@linbit.com>

---

Neil: you may want to double check linear.c and raid*.c for similar logic,
even though it is unlikely that someone puts md raid6 on top of something
exposing a merge_bvec_fn.

This is not the first time this has been patched, btw.
See https://bugzilla.redhat.com/show_bug.cgi?id=440093
and the patch by Mikulas:
https://bugzilla.redhat.com/attachment.cgi?id=342638&action=diff

---
drivers/md/dm-table.c | 12 ++++++++++--
1 files changed, 10 insertions(+), 2 deletions(-)

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);
--
1.6.3.3