linux-kernel - Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations

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Message-ID: <56156FAF.9020002@arm.com>
Date:	Wed, 7 Oct 2015 20:17:03 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"arnd@...db.de" <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"sakari.ailus@....fi" <sakari.ailus@....fi>,
	"sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"m.szyprowski@...sung.com" <m.szyprowski@...sung.com>
Subject: Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations

On 29/09/15 22:27, Andrew Morton wrote:
[...]
> If I'm understanding things correctly, some allocators zero the memory
> by default and others do not.  And we have an unknown number of drivers
> which are assuming that the memory is zeroed.
>
> Correct?

That's precisely the motivation here, yes.

> If so, our options are
>
> a) audit all callers, find the ones which expect zeroed memory but
>     aren't passing __GFP_ZERO and fix them.
>
> b) convert all allocators to zero the memory by default.
>
> Obviously, a) is better.  How big a job is it?

This I'm not so sure of, hence the very tentative first step. For a very 
crude guess at an an upper bound:

$ git grep -E '(dma|pci)_alloc_co(her|nsist)ent' drivers/ | wc -l
1148

vs.

$ git grep -E '(dma|pci)_zalloc_co(her|nsist)ent' drivers/ | wc -l
234

noting that the vast majority of the former are still probably benign, 
but picking out those which aren't from the code alone without knowledge 
of and/or access to the hardware might be non-trivial.

> This patch will help the process, if people use it.
>
>>>> +	if (IS_ENABLED(CONFIG_DMA_API_DEBUG_POISON) && !(flags & __GFP_ZERO))
>>>> +		memset(virt, DMA_ALLOC_POISON, size);
>>>> +
>>>
>>> This is likely to be slow in the case of non-cached memory and large
>>> allocations.  The config option should come with a warning.
>>
>> It depends on DMA_API_DEBUG, which already has a stern performance
>> warning, is additionally hidden behind EXPERT, and carries a slightly
>> flippant yet largely truthful warning that actually using it could break
>> pretty much every driver in your system; is that not enough?
>
> It might be helpful to provide a runtime knob as well - having to
> rebuild&reinstall just to enable/disable this feature is a bit painful.

Good point - there's always the global DMA debug disable knob, but this 
particular feature probably does warrant finer-grained control to be 
really practical. Having thought about it some more, it's also probably 
wrong that this doesn't respect the dma_debug_driver filter, given that 
it is actually invasive; in fixing that, how about if it also *only* 
applied when a specific driver is filtered? Then there would be no 
problematic "break anything and everything" mode, and the existing 
debugfs controls should suffice.

Robin.

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