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block: fix nr_phys_segments miscalculation bug 

This fixes the bug reported by Nikanth Karthikesan <knikanth@suse.de>:

http://lkml.org/lkml/2008/10/2/203

The root cause of the bug is that blk_phys_contig_segment
miscalculates q->max_segment_size.

blk_phys_contig_segment checks:

req->biotail->bi_size + next_req->bio->bi_size > q->max_segment_size

But blk_recalc_rq_segments might expect that req->biotail and the
previous bio in the req are supposed be merged into one
segment. blk_recalc_rq_segments might also expect that next_req->bio
and the next bio in the next_req are supposed be merged into one
segment. In such case, we merge two requests that can't be merged
here. Later, blk_rq_map_sg gives more segments than it should.

We need to keep track of segment size in blk_recalc_rq_segments and
use it to see if two requests can be merged. This patch implements it
in the similar way that we used to do for hw merging (virtual
merging).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
This commit is contained in:
FUJITA Tomonori 2008-10-13 14:19:05 +02:00 committed by Jens Axboe
parent 0fc71e3d65
commit 8677142710
2 changed files with 25 additions and 2 deletions
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20
block/blk-merge.c
View File

@ -77,12 +77,20 @@ void blk_recalc_rq_segments(struct request *rq)
continue;
}
new_segment:
if (nr_phys_segs == 1 && seg_size > rq->bio->bi_seg_front_size)
rq->bio->bi_seg_front_size = seg_size;
nr_phys_segs++;
bvprv = bv;
seg_size = bv->bv_len;
highprv = high;
}
if (nr_phys_segs == 1 && seg_size > rq->bio->bi_seg_front_size)
rq->bio->bi_seg_front_size = seg_size;
if (seg_size > rq->biotail->bi_seg_back_size)
rq->biotail->bi_seg_back_size = seg_size;
rq->nr_phys_segments = nr_phys_segs;
}
@ -106,7 +114,8 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
if (!test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags))
return 0;
if (bio->bi_size + nxt->bi_size > q->max_segment_size)
if (bio->bi_seg_back_size + nxt->bi_seg_front_size >
q->max_segment_size)
return 0;
if (!bio_has_data(bio))
@ -309,6 +318,8 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
struct request *next)
{
int total_phys_segments;
unsigned int seg_size =
req->biotail->bi_seg_back_size + next->bio->bi_seg_front_size;
/*
* First check if the either of the requests are re-queued
@ -324,8 +335,13 @@ 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))
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;
total_phys_segments--;
}
if (total_phys_segments > q->max_phys_segments)
return 0;

7
include/linux/bio.h
View File

@ -79,6 +79,13 @@ struct bio {
unsigned int bi_size; /* residual I/O count */
/*
* To keep track of the max segment size, we account for the
* sizes of the first and last mergeable segments in this bio.
*/
unsigned int bi_seg_front_size;
unsigned int bi_seg_back_size;
unsigned int bi_max_vecs; /* max bvl_vecs we can hold */
unsigned int bi_comp_cpu; /* completion CPU */