Re: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw_segments

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Oct 07 2008, FUJITA Tomonori wrote:
> On Thu, 2 Oct 2008 19:13:57 +0200
> Jens Axboe <jens.axboe@oracle.com> wrote:
> 
> > On Thu, Oct 02 2008, James Bottomley wrote:
> > > On Thu, 2008-10-02 at 18:58 +0200, Jens Axboe wrote:
> > > > On Thu, Oct 02 2008, James Bottomley wrote:
> > > > > The bug would appear to be that we sometimes only look at q->max_sectors
> > > > > when deciding on mergability.  Either we have to insist on max_sectors
> > > > > <= hw_max_sectors, or we have to start using min(q->max_sectors,
> > > > > q->max_hw_sectors) for this.
> > > > 
> > > > q->max_sectors MUST always be <= q->max_hw_sectors, otherwise we could
> > > > be sending down requests that are too large for the device to handle. So
> > > > that condition would be a big bug. The sysfs interface checks for this,
> > > > and blk_queue_max_sectors() makes sure that is true as well.
> > > 
> > > Yes, that seems always to be enforced.  Perhaps there are other ways of
> > > tripping this problem ... I'm still sure if it occurs it's because we do
> > > a physical merge where a virtual merge is forbidden.
> > > 
> > > > The fixes proposed still look weird. There is no phys vs hw segment
> > > > constraints, the request must adhere to the limits set by both. It's
> > > > mostly a moot point anyway, as 2.6.28 will get rid of the hw accounting
> > > > anyway.
> > > 
> > > Agree all three proposed fixes look wrong.  However, if it's what I
> > > think, just getting rid of hw accounting won't fix the problem because
> > > we'll still have the case where a physical merge is forbidden by iommu
> > > constraints ... this still needs to be accounted for.
> > > 
> > > What we really need is a demonstration of what actually is going
> > > wrong ...
> > 
> > Yep, IIRC we both asked for that the last time as well... Nikanth?
> 
> Possibly, blk_phys_contig_segment might miscalculate
> q->max_segment_size?
> 
> blk_phys_contig_segment does:
> 
> req->biotail->bi_size + next_req->bio->bi_size > q->max_segment_size;
> 
> But it's possible that req->biotail and the previous bio are supposed
> be merged into one segment? Then we could create too large segment
> here.

Hmm yes, that looks like it could indeed be a problem! We could fix this
with similar logic to what we used to do for the hw merging, keep track
of the current segment size that this bio belongs to, so it would end up
ala

        if (blk_phys_contig_segment(q, req->biotail, next->bio) &&
            rq->biotail->bi_seg_size + next->bio->bi_size <= q->max_segment_size) {
                total_phys_segments--;
                next->bio->bi_seg_size = rq->biotail->bi_seg_size + next->bio->bi_size;
        }

for the merge part.

-- 
Jens Axboe