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

drivers/md/linear.c

@@ -172,12 +172,14 @@ static linear_conf_t *linear_conf(mddev_t *mddev, int raid_disks)
 		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.
 		 */
-		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);
+		}
 
 		conf->array_sectors += rdev->sectors;
 		cnt++;

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++;

drivers/md/raid0.c

@@ -176,14 +176,15 @@ static int create_strip_zones(mddev_t *mddev)
 		disk_stack_limits(mddev->gendisk, rdev1->bdev,
 				  rdev1->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.
 		 */
 
-		if (rdev1->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 (rdev1->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 (!smallest || (rdev1->sectors < smallest->sectors))
 			smallest = rdev1;
 		cnt++;

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;

drivers/md/raid10.c

@@ -1155,13 +1155,17 @@ static int raid10_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;
@@ -2255,12 +2259,14 @@ 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.
 		 */
-		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);
+		}
 
 		disk->head_position = 0;
 	}