[PATCH for-4.4] block: split bios to max possible length

[Keith Busch]

This allows bio splits in the middle of a vector to form the largest
possible bio at the h/w's desired alignment, and guarantees the bio being
split will have some data. Previously, if the first vector's length was
greater than the allowable amount, the bio would split at a zero length
and hit a kernel BUG.

The length check is moved after the SG_GAPS check so that check doesn't
need to be duplicated in the split case.

Fixes: d3805611130af9b911e908af9f67a3f64f4f0914
bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=110231

Signed-off-by: Keith Busch <keith.busch at intel.com>
---
 block/blk-merge.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index e73846a..e886a7d 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -81,9 +81,6 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 	struct bio *new = NULL;
 
 	bio_for_each_segment(bv, bio, iter) {
-		if (sectors + (bv.bv_len >> 9) > blk_max_size_offset(q, bio->bi_iter.bi_sector))
-			goto split;
-
 		/*
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.
@@ -91,6 +88,17 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
 			goto split;
 
+		if (sectors + (bv.bv_len >> 9) > blk_max_size_offset(q, bio->bi_iter.bi_sector)) {
+			/*
+			 * Consider this a new segment if we're taking any part
+			 * of this vector.
+			 */
+			if (sectors < blk_max_size_offset(q, bio->bi_iter.bi_sector))
+				++nsegs;
+			sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
+			goto split;
+		}
+
 		if (bvprvp && blk_queue_cluster(q)) {
 			if (seg_size + bv.bv_len > queue_max_segment_size(q))
 				goto new_segment;
-- 
2.6.2.307.g37023ba