linux-kernel - Re: [PATCH v10 13/17] mm: vmalloc: add flag preventing guard hole allocation

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Date:	Fri, 30 Jan 2015 20:51:24 +0300
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
	Konstantin Serebryany <kcc@...gle.com>,
	Dmitry Chernenkov <dmitryc@...gle.com>,
	Andrey Konovalov <adech.fo@...il.com>,
	Yuri Gribov <tetra2005@...il.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Christoph Lameter <cl@...ux.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v10 13/17] mm: vmalloc: add flag preventing guard hole
 allocation

On 01/30/2015 02:12 AM, Andrew Morton wrote:
> On Thu, 29 Jan 2015 18:11:57 +0300 Andrey Ryabinin <a.ryabinin@...sung.com> wrote:
> 
>> For instrumenting global variables KASan will shadow memory
>> backing memory for modules. So on module loading we will need
>> to allocate shadow memory and map it at exact virtual address.
> 
> I don't understand.  What does "map it at exact virtual address" mean?
> 

I mean that if module_alloc() returned address x, than
shadow memory should be mapped exactly at address kasan_mem_to_shadow(x).

>> __vmalloc_node_range() seems like the best fit for that purpose,
>> except it puts a guard hole after allocated area.
> 
> Why is the guard hole a problem?
> 

Because of guard hole in shadow some future allocations of shadow memory
will fail. Requested address ( kasan_mem_to_shadow(x) ) will be already occupied
by guard hole of previous allocation.


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