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. -- 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>
