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

[Dmitry Vyukov]

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?
It seems that memory for all page_ext is preallocated along with page
structs; but just the lookup is slower.