Re: [PATCH v1 00/11] mm/kasan: support per-page shadow memory to reduce memory consumption

[Vladimir Murzin]

On 30/05/17 09:31, Vladimir Murzin wrote:
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> On 30/05/17 09:15, Dmitry Vyukov wrote:
>> On Tue, May 30, 2017 at 9:58 AM, Vladimir Murzin
>> <vladimir.murzin@xxxxxxx> wrote:
>>> On 29/05/17 16:29, Dmitry Vyukov wrote:
>>>> I have an alternative proposal. It should be conceptually simpler and
>>>> also less arch-dependent. But I don't know if I miss something
>>>> important that will render it non working.
>>>> Namely, we add a pointer to shadow to the page struct. Then, create a
>>>> slab allocator for 512B shadow blocks. Then, attach/detach these
>>>> shadow blocks to page structs as necessary. It should lead to even
>>>> smaller memory consumption because we won't need a whole shadow page
>>>> when only 1 out of 8 corresponding kernel pages are used (we will need
>>>> just a single 512B block). I guess with some fragmentation we need
>>>> lots of excessive shadow with the current proposed patch.
>>>> This does not depend on TLB in any way and does not require hooking
>>>> into buddy allocator.
>>>> The main downside is that we will need to be careful to not assume
>>>> that shadow is continuous. In particular this means that this mode
>>>> will work only with outline instrumentation and will need some ifdefs.
>>>> Also it will be slower due to the additional indirection when
>>>> accessing shadow, but that's meant as "small but slow" mode as far as
>>>> I understand.
>>>>
>>>> But the main win as I see it is that that's basically complete support
>>>> for 32-bit arches. People do ask about arm32 support:
>>>> https://groups.google.com/d/msg/kasan-dev/Sk6BsSPMRRc/Gqh4oD_wAAAJ
>>>> https://groups.google.com/d/msg/kasan-dev/B22vOFp-QWg/EVJPbrsgAgAJ
>>>> and probably mips32 is relevant as well.
>>>> Such mode does not require a huge continuous address space range, has
>>>> minimal memory consumption and requires minimal arch-dependent code.
>>>> Works only with outline instrumentation, but I think that's a
>>>> reasonable compromise.
>>>
>>> .. or you can just keep shadow in page extension. It was suggested back in
>>> 2015 [1], but seems that lack of stack instrumentation was "no-way"...
>>>
>>> [1] https://lkml.org/lkml/2015/8/24/573
>>
>> Right. It describes basically the same idea.
>>
>> How is page_ext better than adding data page struct?
> 
> page_ext is already here along with some other debug options ;)
> 
>> It seems that memory for all page_ext is preallocated along with page
>> structs; but just the lookup is slower.
>>
> 
> Yup. Lookup would look like (based on v4.0):
> 
> ...
> page_ext = lookup_page_ext_begin(virt_to_page(start));
> 
> do {
>         page_ext->shadow[idx++] = value;
> } while (idx < bound);
> 
> lookup_page_ext_end((void *)page_ext);
> 
> ...

Correction: please, ignore that *_{begin,end} stuff - mainline only
lookup_page_ext() is only used.

Cheers
Vladimir

> 
> Cheers
> Vladimir
> 
> 
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