From: Mikulas Patocka A bio that has two or more vector entries, size less than or equal to page size, that crosses a stripe boundary of an underlying md device is accepted by device mapper (it conforms to all its limits) but not by the underlying device. The fix is: If device mapper selects the one-page maximum request size, it also needs to set its own q->merge_bvec_fn to reject any bios with multiple vector entries that span more pages. The problem was discovered in the following scenario: * MD - RAID-0 * LV on the top of it (raid1, snapshot or striped with chunk size/stripe larger than RAID-0 stripe) * one of the logical volumes is exported to xen domU * inside xen domU it is partitioned, the key point is that the partition must be unaligned on page boundary (fdisk normally aligns the partition to 63 sectors which will trigger it) * install the system on the partitioned disk in domU This causes I/O failures in dom0. Reference: https://bugzilla.redhat.com/show_bug.cgi?id=223947 Signed-off-by: Mikulas Patocka Signed-off-by: Alasdair G Kergon --- drivers/md/dm.c | 10 ++++++++++ 1 file changed, 10 insertions(+) Index: linux-2.6.30/drivers/md/dm.c =================================================================== --- linux-2.6.30.orig/drivers/md/dm.c +++ linux-2.6.30/drivers/md/dm.c @@ -925,6 +925,16 @@ static int dm_merge_bvec(struct request_ */ if (max_size && ti->type->merge) max_size = ti->type->merge(ti, bvm, biovec, max_size); + /* + * If the target doesn't support merge method and some of the devices + * provided their merge_bvec method (we know this by looking at + * queue_max_hw_sectors), then we can't allow bios with multiple vector + * entries. So always set max_size to 0, and the code below allows + * just one page. + */ + else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9) + + max_size = 0; out_table: dm_table_put(map);