linux-kernel - Re: [PATCH] BUG: nr_phys_segments cannot be less than nr_hw

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Date:	Fri, 10 Oct 2008 21:32:22 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jens.axboe@...cle.com
Cc:	fujita.tomonori@....ntt.co.jp,
	James.Bottomley@...senPartnership.com, knikanth@...e.de,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH] BUG: nr_phys_segments cannot be less than
 nr_hw_segments

On Fri, 10 Oct 2008 14:03:44 +0200
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Tue, Oct 07 2008, FUJITA Tomonori wrote:
> > On Thu, 2 Oct 2008 19:13:57 +0200
> > Jens Axboe <jens.axboe@...cle.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!

I think so.


> 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

Yeah, exactly.

You want a fix for this 2.6.28? Or disable this feature for 2.6.28?


>         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.
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