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[dm-devel] [PATCH] dm: Deal with merge_bvec_fn in component devices better

This is analogous to commit 627a2d3c29427637f4c5d31ccc7fcbd8d312cd71
('md: deal with merge_bvec_fn in component devices better.'),
which does the same for md-devices at the top of the stack.  The
following explanation is taken from that commit.  Thanks to Neil Brown
<neilb suse de> for the advice.

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.

References: http://bugs.debian.org/604457
Signed-off-by: Ben Hutchings <ben decadent org uk>
We've been carrying this in Debian for about 3 years and it's about time
it got reviewed...  It's basically Neil's work so should probably have
his sign-off as well.


--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -536,13 +536,15 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
 		       (unsigned long long) start << SECTOR_SHIFT);
-	 * Check if merge fn is supported.
-	 * If not we'll force DM to use PAGE_SIZE or
-	 * smaller I/O, just to be safe.
+	 * If we don't call merge_bvec_fn, we must never risk
+	 * violating it, so limit max_phys_segments to 1 lying within
+	 * a single page.
-	if (dm_queue_merge_is_compulsory(q) && !ti->type->merge)
-		blk_limits_max_hw_sectors(limits,
-					  (unsigned int) (PAGE_SIZE >> 9));
+	if (dm_queue_merge_is_compulsory(q) && !ti->type->merge) {
+		limits->max_segments = 1;
+		limits->seg_boundary_mask = PAGE_CACHE_SIZE - 1;
+	}
 	return 0;

Ben Hutchings
The most exhausting thing in life is being insincere. - Anne Morrow Lindberg

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