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Date: Fri, 30 Jan 2015 20:51:24 +0300
From: Andrey Ryabinin <a.ryabinin@samsung.com>
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, Dmitry Vyukov <dvyukov@google.com>,
        Konstantin Serebryany <kcc@google.com>,
        Dmitry Chernenkov <dmitryc@google.com>,
        Andrey Konovalov <adech.fo@gmail.com>,
        Yuri Gribov <tetra2005@gmail.com>,
        Konstantin Khlebnikov <koct9i@gmail.com>,
        Sasha Levin <sasha.levin@oracle.com>, Christoph Lameter <cl@linux.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Dave Hansen <dave.hansen@intel.com>, Andi Kleen <andi@firstfloor.org>,
        x86@kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH v10 13/17] mm: vmalloc: add flag preventing guard hole
 allocation
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On 01/30/2015 02:12 AM, Andrew Morton wrote:
> On Thu, 29 Jan 2015 18:11:57 +0300 Andrey Ryabinin <a.ryabinin@samsung.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.