linux-kernel - Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

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Message-Id: <20080715114058A.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 15 Jul 2008 11:40:46 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mpatocka@...hat.com
Cc:	fujita.tomonori@....ntt.co.jp, davem@...emloft.net,
	andi@...stfloor.org, sparclinux@...r.kernel.org,
	linux-kernel@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

On Mon, 14 Jul 2008 17:26:38 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> On Tue, 15 Jul 2008, FUJITA Tomonori wrote:
> 
> > On Sun, 13 Jul 2008 20:20:35 -0700 (PDT)
> > David Miller <davem@...emloft.net> wrote:
> >
> >> From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
> >> Date: Mon, 14 Jul 2008 11:19:02 +0900
> >>
> >>> Now IOMMUs don't ignore the max_segment_size. We hit this problem.
> >>
> >> It seems simply that max_segment_size needs to propagate
> >> down into the block layer VMERGE code so that it can
> >> act accordingly.
> >
> > Yeah, I think so. I'll take care of that.
> >
> > But I think that even with the fix, SPARC64 IOMMU still hits the
> > problem.
> >
> > With the VMERGE accounting enabled,
> >
> > - an IOMMU must merge segments that the block layer expects the IOMMU
> > to merge.
> >
> > - an IOMMU must return an error if it can't merge such segments.
> >
> > But the current SPARC64 IOMMU code doesn't return an error if it can't
> > merge such segments (as POWER's IOMMU does, as you know).
> 
> BTW. what should the block device driver do when it receives a mapping 
> error? (if it aborts the request and it was write request, there will be 
> data corruption).

I'm not sure how a aborted request can corrupt data on disk.


> Is it even legal to return IOMMU error in case where no 
> real hardware error or overflow happened?

Of course, it's legal. It's a common event like kinda OOM. It's very
possible with old IOMMUs that have the small I/O space. A SCSI driver
retries a failed request later. But note that some drivers are still
not able to handle that.

http://marc.info/?l=linux-kernel&m=121300637114672&w=2
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