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

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Date:	Mon, 27 Oct 2008 19:21:41 +1100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	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

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.

    J
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