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

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Date:	Fri, 10 Oct 2008 21:49:12 +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 14:37:19 +0200
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Fri, Oct 10 2008, FUJITA Tomonori wrote:
> > 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?
> 
> Lets fix it. It wont be part of the initial merge, since it'll need some
> dedicated testing, but we can get it there for 2.6.28. Shall I interpret
> your message as willingness to write up the fix? :)

Yeah, it's on this weekend todo list. :) I want to look at the code
again and make sure I correctly understand the problem.
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