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

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Message-Id: <20080716093456J.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 16 Jul 2008 09:34:36 +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 11:46:46 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> >> 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.
> 
> Because block layer code doesn't know anything about the device, pci 
> access restrictions and so on.

Not true, the block layer knows about the device restrictions like DMA
boundary.

But it's not the point here because the boundary restriction doesn't
matter for the VMERGE accounting. An IOMMU just returns an error if it
can't allocate an I/O space fit for the device restrictions.


Please give me an example how the boundary restriction of a device can
break the VMERGE accounting and an IOMMU if you aren't still sure.
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