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Subject[PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly
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From: Nikanth Karthikesan <knikanth@suse.de>

ll_merge_requests_fn() decreases the req->nr_phys_segments if
blk_phys_contig_segment() returns true, but it is perfectly possible that
blk_hw_contig_segment() is false. A new hw_segment implies a new phys_segment.
So decrementing nr_phys_segments wrongly here triggers the BUG_ON() in
scsi_init_sgtable(), as blk_rq_map_sg() would map 1 phys_segment more than
req->nr_phys_segment. This is easily reproducible. Other callers of
blk_rq_map_sg() should also be affected by this bug.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
---
block/blk-merge.c | 10 ++++------
1 files changed, 4 insertions(+), 6 deletions(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7..6e6d04b 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -392,11 +392,6 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
return 0;

total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
- if (blk_phys_contig_segment(q, req->biotail, next->bio))
- total_phys_segments--;
-
- if (total_phys_segments > q->max_phys_segments)
- return 0;

total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
@@ -410,9 +405,12 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
if (next->nr_hw_segments == 1)
next->biotail->bi_hw_back_size = len;
total_hw_segments--;
+ if (blk_phys_contig_segment(q, req->biotail, next->bio))
+ total_phys_segments--;
}

- if (total_hw_segments > q->max_hw_segments)
+ if (total_phys_segments > q->max_phys_segments ||
+ total_hw_segments > q->max_hw_segments)
return 0;

/* Merge is OK... */










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