Re: [PATCH v1 00/11] mm/kasan: support per-page shadow memory to reduce memory consumption

[Andrey Ryabinin]

On 05/16/2017 04:16 AM, 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.

I'd start from implementing this on the kernel side only. With KASAN_OUTLINE and disabled
stack instrumentation (--param asan-stack=0) it's doable without any changes in gcc.


...
> base vs patched
> 
> MemTotal: 858 MB vs 987 MB
> runtime: 0 MB vs 30MB
> Net Available: 858 MB vs 957 MB
> 
> For 4096 MB QEMU system
> 
> MemTotal: 3477 MB vs 4000 MB
> runtime: 0 MB vs 50MB
> 
> base vs patched (2048 MB QEMU system)
> 204 s vs 224 s
> Net Available: 3477 MB vs 3950 MB
> 
> Thanks.