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SubjectRe: [PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly
On Tue, Sep 23 2008, Nikanth Karthikesan wrote:
>
> 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--;

This is totally broken, I gather you are thinking the bug is fixed since
the counts always match up. Well that's mainly because you know don't do
ANY merging.

> - if (total_phys_segments > q->max_phys_segments)
> - return 0;

Hmm?

I think you should try and describe the problem you are seeing instead,
then we can perhaps find the real issue.

--
Jens Axboe



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