Re: [BUG] 2.6.29-rc6-2450cf in scsi_lib.c (was: Large amount of scsi-sgpool)objects

Jens Axboe
2009-03-05T10:14:39Z

On Thu, Mar 05 2009, Jens Axboe wrote:
> On Thu, Mar 05 2009, FUJITA Tomonori wrote:
> > Oops, somehow I forgot to CC Jens...
> >
> > On Thu, 5 Mar 2009 17:39:17 +0900
> > FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> >
> > > On Thu, 5 Mar 2009 17:36:13 +0900
> > > FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> > >
> > > > CC'ed Jens,
> > > >
> > > > On Wed, 04 Mar 2009 22:56:29 +0000
> > > > James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
> > > >
> > > > > On Wed, 2009-03-04 at 22:45 +0100, Thomas Gleixner wrote:
> > > > > > On Wed, 4 Mar 2009, Thomas Gleixner wrote:
> > > > > >
> > > > > > Instrumented the code and the result of the failing request is
> > > > > > below. Looks like the function which sets up the request gets
> > > > > > nr_phys_segments wrong by one.
> > > > > >
> > > > > > If you need further trace data feel free to ask.
> > > > >
> > > > > OK, the mapping all checks out correctly ... there must be something
> > > > > wrong with the way we count before mapping.
> > > >
> > > > Yeah, looks we miscalculate nr_phys_segments in the merging path.
> > > >
> > > > blk_recount_segments() needs to set bi_seg_front_size and
> > > > bi_seg_back_size for ll_merge_requests_fn()?
> > > >
> > > > =
> > > > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > > > index a104593..efb65b6 100644
> > > > --- a/block/blk-merge.c
> > > > +++ b/block/blk-merge.c
> > > > @@ -111,12 +111,19 @@ void blk_recalc_rq_segments(struct request *rq)
> > > >
> > > > void blk_recount_segments(struct request_queue *q, struct bio *bio)
> > > > {
> > > > + unsigned int seg_size;
> > > > struct bio *nxt = bio->bi_next;
> > > >
> > > > bio->bi_next = NULL;
> > > > - bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, NULL);
> > > > + bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, &seg_size);
> > > > bio->bi_next = nxt;
> > > > bio->bi_flags |= (1 << BIO_SEG_VALID);
> > > > +
> > > > + if (bio->bi_phys_segments == 1 && seg_size > bio->bi_seg_front_size)
> > > > + bio->bi_seg_front_size = seg_size;
> > > > + if (bio->bi_phys_segments > bio->bi_seg_back_size)
> > > > + bio->bi_seg_back_size = seg_size;
> > > > +
> > > > }
> > > > EXPORT_SYMBOL(blk_recount_segments);
> > >
> > > Duh, here's the proper patch.
> > >
> > > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > > index a104593..06e0db4 100644
> > > --- a/block/blk-merge.c
> > > +++ b/block/blk-merge.c
> > > @@ -111,12 +111,19 @@ void blk_recalc_rq_segments(struct request *rq)
> > >
> > > void blk_recount_segments(struct request_queue *q, struct bio *bio)
> > > {
> > > + unsigned int seg_size;
> > > struct bio *nxt = bio->bi_next;
> > >
> > > bio->bi_next = NULL;
> > > - bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, NULL);
> > > + bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, &seg_size);
> > > bio->bi_next = nxt;
> > > bio->bi_flags |= (1 << BIO_SEG_VALID);
> > > +
> > > + if (bio->bi_phys_segments == 1 && seg_size > bio->bi_seg_front_size)
> > > + bio->bi_seg_front_size = seg_size;
> > > + if (seg_size > bio->bi_seg_back_size)
> > > + bio->bi_seg_back_size = seg_size;
> > > +
> > > }
> > > EXPORT_SYMBOL(blk_recount_segments);
>
> Good catch, I merged it with a slight change of layout and clearing
> seg_size initially, to avoid gcc silly errors.

While merging that, I think we can do better than this. Essentially we
just need to have __blk_recalc_rq_segments() track the back bio as well,
then we don't have to pass in a pointer for segment sizes.

Totally untested, comments welcome...

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 6be3797..5a244f0 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -39,14 +39,13 @@ void blk_recalc_rq_sectors(struct request *rq, int nsect)
}

static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
- struct bio *bio,
- unsigned int *seg_size_ptr)
+ struct bio *bio)
{
unsigned int phys_size;
struct bio_vec *bv, *bvprv = NULL;
int cluster, i, high, highprv = 1;
unsigned int seg_size, nr_phys_segs;
- struct bio *fbio;
+ struct bio *fbio, *bbio;

if (!bio)
return 0;
@@ -87,41 +86,28 @@ new_segment:
seg_size = bv->bv_len;
highprv = high;
}
+ bbio = bio;
}

- if (seg_size_ptr)
- *seg_size_ptr = seg_size;
+ if (nr_phys_segs == 1 && seg_size > fbio->bi_seg_front_size)
+ fbio->bi_seg_front_size = seg_size;
+ if (seg_size > bbio->bi_seg_back_size)
+ bbio->bi_seg_back_size = seg_size;

return nr_phys_segs;
}

void blk_recalc_rq_segments(struct request *rq)
{
- unsigned int seg_size = 0, phys_segs;
-
- phys_segs = __blk_recalc_rq_segments(rq->q, rq->bio, &seg_size);
-
- if (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 = phys_segs;
+ rq->nr_phys_segments = __blk_recalc_rq_segments(rq->q, rq->bio);
}

void blk_recount_segments(struct request_queue *q, struct bio *bio)
{
struct bio *nxt = bio->bi_next;
- unsigned int seg_size = 0;

bio->bi_next = NULL;
- bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, NULL);
-
- if (bio->bi_phys_segments == 1 && seg_size > bio->bi_seg_front_size)
- bio->bi_seg_front_size = seg_size;
- if (seg_size > bio->bi_seg_back_size)
- bio->bi_seg_back_size = seg_size;
-
+ bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio);
bio->bi_next = nxt;
bio->bi_flags |= (1 << BIO_SEG_VALID);
}

--
Jens Axboe