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

[Vladimir Murzin]

On 30/05/17 09:49, Dmitry Vyukov wrote:
> On Tue, May 30, 2017 at 10:40 AM, Vladimir Murzin
> <vladimir.murzin@xxxxxxx> wrote:
>> On 30/05/17 09:31, Vladimir Murzin wrote:
>>> [This sender failed our fraud detection checks and may not be who they appear to be. Learn about spoofing at http://aka.ms/LearnAboutSpoofing]
>>>
>>> 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 ;)
> 
> 
> But page struct is also here. What am I missing?
> 

Probably, free room in page struct? I guess most of the page_ext stuff would
love to live in page struct, but... for instance, look at page idle tracking
which has to live in page_ext only for 32-bit.

> 
>>>> 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.
> 
> 
> Note that this added code will be executed during handling of each and
> every memory access in kernel. Every instruction matters on that path.

I know, I know... still better than nothing.

> The additional indirection via page struct will also slow down it, but
> that's the cost for lower memory consumption and potentially 32-bit
> support. For page_ext it looks like even more overhead for no gain.
> 

eefa864 (mm/page_ext: resurrect struct page extending code for debugging)
express some cases where keeping data in page_ext has benefit.

Cheers
Vladimir