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

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, May 30, 2017 at 11:39 AM, Vladimir Murzin
<vladimir.murzin@arm.com> wrote:
>
> On 30/05/17 10:26, Dmitry Vyukov wrote:
> > On Tue, May 30, 2017 at 11:08 AM, Vladimir Murzin
> > <vladimir.murzin@arm.com> wrote:
> >>> <vladimir.murzin@arm.com> 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@arm.com> 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.
> >
> >
> > Sorry for my ignorance. What's the fundamental problem with just
> > pushing everything into page struct?
>
> I think [1] has an answer for your question ;)

It also has an answer for why we should put it into page struct :)


>
> >
> > I don't see anything relevant in page struct comment. Nor I see "idle"
> > nor "tracking" page struct. I see only 2 mentions of CONFIG_64BIT, but
> > both declare the same fields just with different types (int vs short).
>
> Right, it is because implementation is based on page flags [1]:
>
> Note, since there is no room for extra page flags on 32 bit, this feature
> uses extended page flags when compiled on 32 bit.
>
>
> [1] https://lwn.net/Articles/565097/
> [2] 33c3fc7 ("mm: introduce idle page tracking")

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