On Wed, May 24, 2017 at 06:31:04PM +0200, Dmitry Vyukov wrote: > On Wed, May 24, 2017 at 8:04 AM, Joonsoo Kim <js1304@xxxxxxxxx> wrote: > >> >> > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > >> >> > > >> >> > Hello, all. > >> >> > > >> >> > This is an attempt to recude memory consumption of KASAN. Please see > >> >> > following description to get the more information. > >> >> > > >> >> > 1. What is per-page shadow memory > >> >> > > >> >> > This patch introduces infrastructure to support per-page shadow memory. > >> >> > Per-page shadow memory is the same with original shadow memory except > >> >> > the granualarity. It's one byte shows the shadow value for the page. > >> >> > The purpose of introducing this new shadow memory is to save memory > >> >> > consumption. > >> >> > > >> >> > 2. Problem of current approach > >> >> > > >> >> > Until now, KASAN needs shadow memory for all the range of the memory > >> >> > so the amount of statically allocated memory is so large. It causes > >> >> > the problem that KASAN cannot run on the system with hard memory > >> >> > constraint. Even if KASAN can run, large memory consumption due to > >> >> > KASAN changes behaviour of the workload so we cannot validate > >> >> > the moment that we want to check. > >> >> > > >> >> > 3. How does this patch fix the problem > >> >> > > >> >> > This patch tries to fix the problem by reducing memory consumption for > >> >> > the shadow memory. There are two observations. > >> >> > > >> >> > >> >> > >> >> I think that the best way to deal with your problem is to increase shadow scale size. > >> >> > >> >> You'll need to add tunable to gcc to control shadow size. I expect that gcc has some > >> >> places where 8-shadow scale size is hardcoded, but it should be fixable. > >> >> > >> >> The kernel also have some small amount of code written with KASAN_SHADOW_SCALE_SIZE == 8 in mind, > >> >> which should be easy to fix. > >> >> > >> >> Note that bigger shadow scale size requires bigger alignment of allocated memory and variables. > >> >> However, according to comments in gcc/asan.c gcc already aligns stack and global variables and at > >> >> 32-bytes boundary. > >> >> So we could bump shadow scale up to 32 without increasing current stack consumption. > >> >> > >> >> On a small machine (1Gb) 1/32 of shadow is just 32Mb which is comparable to yours 30Mb, but I expect it to be > >> >> much faster. More importantly, this will require only small amount of simple changes in code, which will be > >> >> a *lot* more easier to maintain. > >> > >> > >> Interesting option. We never considered increasing scale in user space > >> due to performance implications. But the algorithm always supported up > >> to 128x scale. Definitely worth considering as an option. > > > > Could you explain me how does increasing scale reduce performance? I > > tried to guess the reason but failed. > > > The main reason is inline instrumentation. Inline instrumentation for > a check of 8-byte access (which are very common in 64-bit code) is > just a check of the shadow byte for 0. For smaller accesses we have > more complex instrumentation that first checks shadow for 0 and then > does precise check based on size/offset of the access + shadow value. > That's slower and also increases register pressure and code size > (which can further reduce performance due to icache overflow). If we > increase scale to 16/32, all accesses will need that slow path. > Another thing is stack instrumentation: larger scale will require > larger redzones to ensure proper alignment. That will increase stack > frames and also more instructions to poison/unpoison stack shadow on > function entry/exit. Now, I see. Thanks for explanation. 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>
