On Tue, May 30, 2017 at 11:08 AM, Vladimir Murzin <vladimir.murzin@xxxxxxx> wrote: >> <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. Sorry for my ignorance. What's the fundamental problem with just pushing everything 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). >>>>> 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>
