linux-kernel - Re: [PATCH 2/2] x86: replace BIO_VMERGE_BOUNDARY with BIOVEC_PHYS

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Date:	Mon, 27 Oct 2008 17:37:47 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jeremy@...p.org
Cc:	fujita.tomonori@....ntt.co.jp, linux-kernel@...r.kernel.org,
	mingo@...e.hu, jens.axboe@...cle.com
Subject: Re: [PATCH 2/2] x86: replace BIO_VMERGE_BOUNDARY with
 BIOVEC_PHYS_MERGEABLE

On Mon, 27 Oct 2008 19:21:41 +1100
Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> FUJITA Tomonori wrote:
> > The block layer always done the physical merge if possible. We don't
> > provide any kernel parameter to disable it.
> >
> > The iommu_bio_merge parameter had been used to enable the virtual
> > merge. As I wrote, the virtual merge feature was completely
> > removed. Effectively, the iommu_bio_merge parameter is meaningless
> > now.
> >   
> 
> Under Xen, pages which appear to be pseudo-physically adjacent are not 
> necessarily really physically adjacent.  We need to hook 
> BIOVEC_PHYS_MERGEABLE to prevent the bio layer from inappropriately 
> merging requests across non-contiguous page boundaries.

I'm not familiar with what Xen does but why can't Xen just override
BIOVEC_PHYS_MERGEABLE?

Why does Xen need to hook BIOVEC_PHYS_MERGEABLE to the iommu_bio_merge
parameter (as this patch does)? BIOVEC_PHYS_MERGEABLE and the
iommu_bio_merge parameter are not related at all.
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