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    Subject[PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly
    Date

    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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