[PATCH] block: fix update first/last mergeable segment sizes

[Wanpeng Li <wanpeng.li@hotmail.com>]

To keep track of the max segment size, the sizes of the first and last 
mergeable segments in the bio are accounted, prev->bio->bi_seg_front_size
and next->biotail->bi_seg_back_size will be updated if blk_phys_contig_segment 
is true, however, there are still some scenarios such like nonconsistent
blk integrity which leads to merge fail, in addtion, futher merge fail is 
incurred due to bio->bi_seg_back_size + next->bi_seg_front_size > 
queue_max_segment_size(q) check in function blk_phys_contig_segment(). This 
patch fix it by updating first/last mergeable segments size iff all the 
conditions of merging are meet.

Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
 block/blk-merge.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 30a0d9f..7e7bda4 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -367,6 +367,7 @@ static int req_gap_to_prev(struct request *req, struct request *next)
 static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 				struct request *next)
 {
+	bool update_segment_size = false;
 	int total_phys_segments;
 	unsigned int seg_size =
 		req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
@@ -391,10 +392,7 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 
 	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
 	if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
-		if (req->nr_phys_segments == 1)
-			req->bio->bi_seg_front_size = seg_size;
-		if (next->nr_phys_segments == 1)
-			next->biotail->bi_seg_back_size = seg_size;
+		update_segment_size = true;
 		total_phys_segments--;
 	}
 
@@ -405,6 +403,12 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
 		return 0;
 
 	/* Merge is OK... */
+	if (update_segment_size) {
+		if (req->nr_phys_segments == 1)
+			req->bio->bi_seg_front_size = seg_size;
+		if (next->nr_phys_segments == 1)
+			next->biotail->bi_seg_back_size = seg_size;
+	}
 	req->nr_phys_segments = total_phys_segments;
 	return 1;
 }
-- 
1.9.1