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

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Message-ID: <20090605182015.GA10342@8bytes.org>
Date:	Fri, 5 Jun 2009 20:20:17 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Torsten Kaiser <just.for.lkml@...glemail.com>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	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 Fri, Jun 05, 2009 at 05:52:27PM +0200, Torsten Kaiser wrote:
> 
> This doesn't look right to me.
> The comment above says "returns the entry from the hash which fits
> best", but this will always return NULL, if there are are multiple
> entrys, but no perfect match.

This is intentional. If there is no perfect-fit there is no way to tell
which entry was meant. So we potentially report wrong errors with a
wrong mapping backtrace which confuses even more than the wrong
"DMA-API: device driver tries to free DMA memory it has not allocated".

> Should there be a warning if more then one possible match were found?

True. That would be better. But I tried to keep the code change as small
as possible without disabling the feature completly.

> * driver maps address 'a' with size 1
> * driver maps same address 'a' with size 2
> * driver wrongly unmaps the second allocation with size 1
>   -> no warning, because the first allocation is returned

Hmm, I am not sure if we can handle this situation correctly in the
dma-debug code. There is no unique key to identify a mapping request
which allows to assign an unmap request to it. Currently dma-debug uses
device and dma-address. But that seems not to be sufficient. The
best-fit algorithm is also a but fuzzy of course.

> * driver wants to correctly unmap the first allocation
>   -> wrong warning about this unmap because size mismatch

Ok, at least we get a warning about a bug. Not very useful because it
reports the wrong bug. Is this the situation which triggered the
original bug report?

> Also what about sg_call_ents and sg_mapped_ents?
> Could it be possible to get the same address/size with different sg-counts.

It looks not forbidden in the API. So I guess this can happen too.

Regards,

	Joerg
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