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 ;) > >> 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 > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
