md: deal with merge_bvec_fn in component devices better.

If a component device has a merge_bvec_fn then as we never call it
we must ensure we never need to.  Currently this is done by setting
max_sector to 1 PAGE, however this does not stop a bio being created
with several sub-page iovecs that would violate the merge_bvec_fn.

So instead set max_segments to 1 and set the segment boundary to the
same as a page boundary to ensure there is only ever one single-page
segment of IO requested at a time.

This can particularly be an issue when 'xen' is used as it is
known to submit multiple small buffers in a single bio.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org

1 files changed, 12 insertions, 8 deletions

diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 4b323f45ad74..5558ebc705c8 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -301,14 +301,16 @@ static int multipath_add_disk(mddev_t *mddev, mdk_rdev_t *rdev)
                                           rdev->data_offset << 9);
 
                 /* as we don't honour merge_bvec_fn, we must never risk
-                 * violating it, so limit ->max_sector to one PAGE, as
-                 * a one page request is never in violation.
+                 * violating it, so limit ->max_segments to one, lying
+                 * within a single page.
                  * (Note: it is very unlikely that a device with
                  * merge_bvec_fn will be involved in multipath.)
                  */
-                        if (q->merge_bvec_fn &&
-                            queue_max_sectors(q) > (PAGE_SIZE>>9))
-                                blk_queue_max_hw_sectors(mddev->queue, PAGE_SIZE>>9);
+                        if (q->merge_bvec_fn) {
+                                blk_queue_max_segments(mddev->queue, 1);
+                                blk_queue_segment_boundary(mddev->queue,
+                                                           PAGE_CACHE_SIZE - 1);
+                        }
 
                         conf->working_disks++;
                         mddev->degraded--;
@@ -476,9 +478,11 @@ static int multipath_run (mddev_t *mddev)
                 /* as we don't honour merge_bvec_fn, we must never risk
                  * violating it, not that we ever expect a device with
                  * a merge_bvec_fn to be involved in multipath */
-                if (rdev->bdev->bd_disk->queue->merge_bvec_fn &&
-                    queue_max_sectors(mddev->queue) > (PAGE_SIZE>>9))
-                        blk_queue_max_hw_sectors(mddev->queue, PAGE_SIZE>>9);
+                if (rdev->bdev->bd_disk->queue->merge_bvec_fn) {
+                        blk_queue_max_segments(mddev->queue, 1);
+                        blk_queue_segment_boundary(mddev->queue,
+                                                   PAGE_CACHE_SIZE - 1);
+                }
 
                 if (!test_bit(Faulty, &rdev->flags))
                         conf->working_disks++;