On Tue, May 30, 2017 at 4:16 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote: > On 05/29/2017 06:29 PM, Dmitry Vyukov wrote: >> Joonsoo, >> >> I guess mine (and Andrey's) main concern is the amount of additional >> complexity (I am still struggling to understand how it all works) and >> more arch-dependent code in exchange for moderate memory win. >> >> Joonsoo, Andrey, >> >> 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. > > It seems that you are forgetting about stack instrumentation. > You'll have to disable it completely, at least with current implementation of it in gcc. > >> 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. > > I don't see how above is relevant for 32-bit arches. Current design > is perfectly fine for 32-bit arches. I did some POC arm32 port couple years > ago - https://github.com/aryabinin/linux/commits/kasan/arm_v0_1 > It has some ugly hacks and non-critical bugs. AFAIR it also super-slow because I (mistakenly) > made shadow memory uncached. But otherwise it works. > >> 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. >> >> What do you think? > > I don't understand why we trying to invent some hacky/complex schemes when we already have > a simple one - scaling shadow to 1/32. It's easy to implement and should be more performant comparing > to suggested schemes. If 32-bits work with the current approach, then I would also prefer to keep things simpler. FWIW clang supports settings shadow scale via a command line flag (-asan-mapping-scale). -- 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>
