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

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Message-Id: <20151007123351.763b98a8fe82a86ce7fdaf0c@linux-foundation.org>
Date:	Wed, 7 Oct 2015 12:33:51 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Robin Murphy <robin.murphy@....com>
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 Wed, 7 Oct 2015 20:17:03 +0100 Robin Murphy <robin.murphy@....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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