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. -- 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>
