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

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  Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations Robin Murphy <robin.murphy@arm.com> - 2015-10-07 21:20 +0200
    Re: [PATCH 4/4] dma-debug: Allow poisoning nonzero allocations Andrew Morton <akpm@linux-foundation.org> - 2015-10-07 21:40 +0200

#1241742 — 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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#1241753

On Wed, 7 Oct 2015 20:17:03 +0100 Robin Murphy <robin.murphy@arm.com> wrote:

> > 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.

Yes, this should respect the driver filtering.

On reflection...

The patch poisons dma buffers if CONFIG_DMA_API_DEBUG and if __GFP_ZERO
wasn't explicitly used.  I'm rather surprised that the dma-debug code
didn't do this from day one.

I'd be inclined to enable this buffer-poisoning by default.  Do you
have a feeling for how much overhead that will add?  Presumably not
much, if __GFP_ZERO is acceptable.

Also, how about we remove CONFIG_DMA_API_DEBUG_POISON and switch to a
debugfs knob?


btw, the documentation could do with a bit of a tune-up.  The comments
in dma-debug.c regarding driver filtering are non-existent. 
Documentation/kernel-parameters.txt says "The filter can be disabled or
changed to another driver later using sysfs" but
Documentation/DMA-API.txt talks about debugfs.
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