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, 17 insertions, 11 deletions

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 5a06122abd3b..f741f77eeb2b 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1152,13 +1152,17 @@ static int raid1_add_disk(mddev_t *mddev, mdk_rdev_t *rdev)
 
                         disk_stack_limits(mddev->gendisk, rdev->bdev,
                                           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.
+                        /* as we don't honour merge_bvec_fn, we must
+                         * never risk violating it, so limit
+                         * ->max_segments to one lying with a single
+                         * page, as a one page request is never in
+                         * violation.
                          */
-                        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);
+                        }
 
                         p->head_position = 0;
                         rdev->raid_disk = mirror;
@@ -2098,12 +2102,14 @@ static int run(mddev_t *mddev)
                 disk_stack_limits(mddev->gendisk, rdev->bdev,
                                   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 1 lying within
+                 * a single page, as a one page request is never in violation.
                  */
-                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);
+                }
         }
 
         mddev->degraded = 0;