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Subject[PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments
Date
This is a follow-up to my earlier mail http://lkml.org/lkml/2008/9/23/294 
([PATCH] BUG: ll_merge_requests_fn() updates req->nr_phys_segments wrongly)

It is possible for the merging code to create lesser no of phys segments than
hw segments, but every hw segment needs atleast one new phys segment. This
triggers the BUG() on scsi_init_sgtable() as blk_rq_map_sg() returns more no
of segments than rq->nr_phys_segments

The following blktrace shows a sequence of bio's to trigger such condition on
my machine with max_sectors_kb=512 & max_hw_sectors_kb=32767.

8,0 0 12 1.943680621 2261 Q R 6184075 + 55 [bash]
8,0 0 13 1.943684671 2261 G R 6184075 + 55 [bash]
8,0 0 14 1.943690189 2261 P N [bash]
8,0 0 15 1.943692075 2261 I R 6184075 + 55 [bash]
8,0 0 16 1.943712119 2261 D R 6184075 + 55 [bash]
8,0 0 17 1.943718684 2261 Q R 6184130 + 55 [bash]
8,0 0 18 1.943721897 2261 G R 6184130 + 55 [bash]
8,0 0 19 1.943726576 2261 P N [bash]
8,0 0 20 1.943727763 2261 I R 6184130 + 55 [bash]
8,0 0 21 1.943731675 2261 Q R 6184241 + 56 [bash]
8,0 0 22 1.943735167 2261 G R 6184241 + 56 [bash]
8,0 0 23 1.943739497 2261 I R 6184241 + 56 [bash]
8,0 0 24 1.943742081 2261 Q R 6184185 + 56 [bash]
8,0 0 25 1.943744875 2261 M R 6184185 + 56 [bash]
8,0 0 26 1.943753535 2261 Q R 6184352 + 55 [bash]
8,0 0 27 1.943756538 2261 G R 6184352 + 55 [bash]
8,0 0 28 1.943760868 2261 I R 6184352 + 55 [bash]
8,0 0 29 1.943763522 2261 Q R 6184407 + 55 [bash]
8,0 0 30 1.943766316 2261 M R 6184407 + 55 [bash]
8,0 0 31 1.943770506 2261 Q R 6184297 + 55 [bash]
8,0 0 32 1.943772951 2261 F R 6184297 + 55 [bash]
8,0 0 33 1.943780843 2261 Q R 6184462 + 55 [bash]
8,0 0 34 1.943783776 2261 M R 6184462 + 55 [bash]
8,0 0 35 1.944444684 0 UT N [swapper] 2
8,0 0 36 1.944453624 10 U N [kblockd/0] 2
8,0 0 37 1.944470595 10 D R 6184130 + 387 [kblockd/0]
8,0 0 38 1.970340853 0 C R 6184075 + 55 [0]
8,0 0 39 1.973576739 0 C R 6184130 + 387 [0]

Patches against Jens git to fix this.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7..44cc1d7 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -395,9 +395,6 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
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)) {
int len = req->biotail->bi_hw_back_size +
@@ -415,6 +412,15 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
if (total_hw_segments > q->max_hw_segments)
return 0;

+ /*
+ * FIXME: if 2 phys segments is used for a single hw segment?
+ */
+ if (total_phys_segments < total_hw_segments)
+ total_phys_segments = total_hw_segments;
+
+ if (total_phys_segments > q->max_phys_segments)
+ return 0;
+
/* Merge is OK... */
req->nr_phys_segments = total_phys_segments;
req->nr_hw_segments = total_hw_segments;
But this may not be the complete fix? I am not sure whether the condition in
FIXME comment can be triggered. The following patch may be a better fix.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7..b2c37f4 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -467,6 +467,8 @@ static int attempt_merge(struct request_queue *q, struct
request *req,

req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

+ blk_recalc_rq_segments(req);
+
elv_merge_requests(q, req, next);

if (req->rq_disk) {
But still wouldn't it be incomplete fix as blk_recalc_rq_segments() can
produce nr_phys_segments > q->max_phys_segments? Do we need something like
this.
Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7..e4431db 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -427,6 +427,8 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
static int attempt_merge(struct request_queue *q, struct request *req,
struct request *next)
{
+ struct bio *req_biotail, *req_biotail_bi_next;
+
if (!rq_mergeable(req) || !rq_mergeable(next))
return 0;

@@ -462,11 +464,21 @@ static int attempt_merge(struct request_queue *q, struct
request *req,
if (time_after(req->start_time, next->start_time))
req->start_time = next->start_time;

+ req_biotail = req->biotail;
+ req_biotail_bi_next = req->biotail->bi_next;
req->biotail->bi_next = next->bio;
req->biotail = next->biotail;

req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

+ blk_recalc_rq_segments(req);
+ if (req->nr_phys_segments > q->max_phys_segments) {
+ req->biotail = req_biotail;
+ req->biotail->bi_next = req_biotail_bi_next;
+ blk_recalc_rq_segments(req);
+ return 0;
+ }
+
elv_merge_requests(q, req, next);

if (req->rq_disk) {
But blk_recalc_rq_segments() cannot increase nr_phys_segments by more than
one. So checking for one lesser limit earlier should also work? But we would
be mostly merging 1 lesser segment.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
---
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7..d9b5289 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -256,7 +256,7 @@ static inline int ll_new_mergeable(struct request_queue
*q,
{
int nr_phys_segs = bio_phys_segments(q, bio);

- if (req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
+ if (req->nr_phys_segments + nr_phys_segs > q->max_phys_segments - 1) {
req->cmd_flags |= REQ_NOMERGE;
if (req == q->last_merge)
q->last_merge = NULL;
@@ -395,7 +395,7 @@ static int ll_merge_requests_fn(struct request_queue *q,
struct request *req,
if (blk_phys_contig_segment(q, req->biotail, next->bio))
total_phys_segments--;

- if (total_phys_segments > q->max_phys_segments)
+ if (total_phys_segments > q->max_phys_segments - 1)
return 0;

total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
@@ -467,6 +467,8 @@ static int attempt_merge(struct request_queue *q, struct
request *req,

req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;

+ blk_recalc_rq_segments(req);
+
elv_merge_requests(q, req, next);

if (req->rq_disk) {

Thanks
Nikanth Karthikesan




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