linux-kernel - Re: [PATCH] kmemleak: Ignore the aperture memory hole on x86

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Date:	Sun, 16 Aug 2009 09:04:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kmemleak: Ignore the aperture memory hole on x86_64


* Catalin Marinas <catalin.marinas@....com> wrote:

> On Sat, 2009-08-15 at 15:17 +0100, Ingo Molnar wrote:
> > * Catalin Marinas <catalin.marinas@....com> wrote:
> > > +     /*
> > > +      * Kmemleak should not scan this block as it may not be mapped via the
> > > +      * kernel direct mapping.
> > > +      */
> > > +     kmemleak_ignore(p);
> > 
> > More importantly, kmemleak should _never_ do the garbage collection
> > scan for device memory (such as the agp aperture above). All the
> > aperture areas are in that category - PCI aperture, IOMMU areas,
> > etc. etc.
> > 
> > Please double check that kmemleak does not check those - there are
> > devices where pure reading of that address space can have
> > side-effects.
> 
> I'll do a grep. But would such memory still be mapped in the 
> kernel direct mapping? [...]

It should not be mapped directly - we try to map all kinds of 
resources 'precisely', so that there can be no cache aliasing 
complications due to over-mapping - but still, there are 
compatibility ranges that are always mapped (the BIOS area for 
example).

> [...] In this particular case, it was alloc_bootmem() memory which 
> seems to have been unmapped (and cause an oops), otherwise, at 
> least on some architectures, may have problems with speculative 
> fetches.
> 
> Kmemleak doesn't track other mappings like ioremap, so it should 
> not scan device memory.
> 
> Since you raised this, I realised there is a class of kmalloc'ed 
> memory blocks that may have some issues on non-coherent 
> architectures. If such blocks are used for DMA and cache 
> invalidation is only done in dma_map_single(FROM_DEVICE) (the ARM 
> case), kmemleak scanning before dma_unmap_single() may pollute the 
> cache. One solution is to invalidate the caches again in 
> dma_unmap_single(). I'm not sure ignoring GFP_DMA blocks would be 
> feasible if this flag is used for other blocks containing 
> pointers. I need to do some tests but I don't think x86 is 
> affected.

Yeah, x86 shouldnt be affected.

	Ingo
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