linux-kernel - Re: [PATCH] dma-debug: disable DMA_API

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Message-ID: <20090607081305.GA12497@elte.hu>
Date:	Sun, 7 Jun 2009 10:13:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lethal@...ux-sh.org, just.for.lkml@...glemail.com,
	hancockrwd@...il.com, jens.axboe@...cle.com, bharrosh@...asas.com,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH] dma-debug: disable DMA_API_DEBUG for now


* Joerg Roedel <joerg.roedel@....com> wrote:

> 
> On Fri, Jun 05, 2009 at 05:33:14PM +0900, FUJITA Tomonori wrote:
> > dma-debugs wrongly assumes that no multiple DMA transfers are
> > performed against the same dma address on one device at the same
> > time. However it's true only with hardware IOMMUs. For example, an
> > application can easily send the same buffer twice with different
> > lengths to one device by using DIO and AIO. If these requests are not
> > unmapped in the same order in which they were mapped,
> > hash_bucket_find() finds a wrong entry and gives a false warning.
> > 
> > We should fix this before 2.6.30 release. Seems that there is no
> > easy way to fix it. I think that it's better to just disable
> > dma-debug for now.
> > 
> > Torsten Kaiser found this bug with the RAID1 configuration:
> > 
> > http://marc.info/?t=124336541900008&r=1&w=2
> > 
> 
> Argh, I never thought that this can happen. But its not explicitly 
> forbidden so we have to handle this situation. Thanks for tracking 
> down the bug to this issue.
>
> However, I think there is a somehow simple fix for the issue. 
> Patch is attached. Its the least intrusive way I can think of to 
> fix this problem.
>
> But its up to Linus/Ingo to decide if it can be accepted at this 
> very late point in the cycle. Since dma-debug is new with 2.6.30 
> it will at least not introduce any regression. [...]

I think it's too late for v2.6.30 to do any of the changes - and the 
DMA debug facility is off by default.

Also, i think such DMA patterns, while 'allowed' can be quite 
dangerous as its such a rare usage combination really. AIO and DIO 
are crazy to begin with, mixing AIO and DIO for the same buffer is 
madness square two. (It can result in 3 agents for the same memory 
address: CPU, dma1 and dma2. How many interesting chipset erratums 
could there be related to such scenarios?)

But it is certainly not the task of a debug facility to restrict 
existing user-visible ABIs, so fixing the false positive is correct.

So i've applied your fix to the iommu branch for v2.6.31 and marked 
it for -stable backporting, that way 2.6.30.1 will be able to pick 
the patch up (if it remains problem-free in testing).

Thanks,

	Ingo
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