On Thu, Jun 01, 2017 at 08:06:02PM +0200, Dmitry Vyukov wrote: > 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). Hello, To confirm the final consensus, I did a quick comparison of scaling approach and mine. Note that scaling approach can be co-exist with mine. And, there is an assumption that we can disable quarantine and other optional feature of KASAN. Scaling vs Mine Memory usage: 1/32 of total memory. vs can be far less than 1/32. Slab object layout: should be changed. vs none. Usability: hard. vs simple. (Updating compiler is not required) Implementation complexity: simple. vs complex. Porting to other ARCH: simple. vs hard (But, not mandatory) So, do both you disagree to merge my per-page shadow? If so, I will not submit v2. Please let me know your decision. Thanks. -- 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>
