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); ... 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>
