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

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Message-ID: <64bb37e0906070122t652051d7oaa1e7ec70e4c05ea@mail.gmail.com>
Date:	Sun, 7 Jun 2009 10:22:00 +0200
From:	Torsten Kaiser <just.for.lkml@...glemail.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lethal@...ux-sh.org, 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

On Sun, Jun 7, 2009 at 10:13 AM, Ingo Molnar <mingo@...e.hu> wrote:
> * Joerg Roedel <joerg.roedel@....com> wrote:
>> 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?)

I think in my case the cause is somewhat simpler: RAID1
My root filesystem is on a RAID1 that consists of two disks, both
connected to the sata_sil24 controller.
So it is only natural for the md driver to issue two dma read requests
for the same address: one for each drive.

Torsten
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