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

[Joonsoo Kim]

On Wed, May 17, 2017 at 03:17:13PM +0300, Andrey Ryabinin wrote:
> 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 agree that it is also a good option to reduce memory consumption.
Nevertheless, there are two reasons that justifies this patchset.

1) With this patchset, memory consumption isn't increased in
proportional to total memory size. Please consider my 4Gb system
example on the below. With increasing shadow scale size to 32, memory
would be consumed by 128M. However, this patchset consumed 50MB. This
difference can be larger if we run KASAN with bigger machine.

2) These two optimization can be applied simulatenously. It is just an
orthogonal feature. If shadow scale size is increased to 32, memory
consumption will be decreased in case of my patchset, too.

Therefore, I think that this patchset is useful in any case.

Note that increasing shadow scale has it's own trade-off. It requires
that the size of slab object is aligned to shadow scale. It will
increase memory consumption due to slab.

Thanks.