linux-kernel - Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq

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Message-Id: <20080716003047P.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 16 Jul 2008 00:30:43 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mpatocka@...hat.com
Cc:	fujita.tomonori@....ntt.co.jp, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	davem@...emloft.net, linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in
 blk_recalc_rq_segments about VMERGE

On Tue, 15 Jul 2008 10:37:58 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> >>> This bug could happen on alpha, parisc, and sparc, which use VMERGE.
> >>
> >> Parisc doesn't use virtual merge accounting (there is variable for it but
> >> it's always 0).
> >
> > Hmm, really? Looks like PARISC IOMMUs (ccio-dma.c and sba_iomm.c) set
> > parisc_vmerge_boundary (CC'ed PARISC mailing list).
> 
> That's right, I looked only at arch and include.
> 
> >> On sparc64 it is broken anyway with or without your patch.
> >
> > Yeah, we need to modify SPARC64 IOMMU code (I'm not sure that it's
> > worth). Right now, the best fix is setting BIO_VMERGE_BOUNDARY to 0.
> 
> Even if we fix it now, the question is: how long it will stay fixed? Until 
> someone makes another change to struct device that restricts boundaries on 
> some wacky hardware.

I'm not sure how the boundary restriction of a device can break
the VMERGE accounting.
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