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 -- 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>
