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

[Joonsoo Kim]

On Wed, May 17, 2017 at 04:23:17PM +0900, Joonsoo Kim wrote:
> > However, I see some very significant slowdowns with inline
> > instrumentation. I did 3 tests:
> > 1. Boot speed, I measured time for a particular message to appear on
> > console. Before:
> > [    2.504652] random: crng init done
> > [    2.435861] random: crng init done
> > [    2.537135] random: crng init done
> > After:
> > [    7.263402] random: crng init done
> > [    7.263402] random: crng init done
> > [    7.174395] random: crng init done
> > 
> > That's ~3x slowdown.
> > 
> > 2. I've run bench_readv benchmark:
> > https://raw.githubusercontent.com/google/sanitizers/master/address-sanitizer/kernel_buildbot/slave/bench_readv.c
> > as:
> > while true; do time ./bench_readv bench_readv 300000 1; done
> > 
> > Before:
> > sys 0m7.299s
> > sys 0m7.218s
> > sys 0m6.973s
> > sys 0m6.892s
> > sys 0m7.035s
> > sys 0m6.982s
> > sys 0m6.921s
> > sys 0m6.940s
> > sys 0m6.905s
> > sys 0m7.006s
> > 
> > After:
> > sys 0m8.141s
> > sys 0m8.077s
> > sys 0m8.067s
> > sys 0m8.116s
> > sys 0m8.128s
> > sys 0m8.115s
> > sys 0m8.108s
> > sys 0m8.326s
> > sys 0m8.529s
> > sys 0m8.164s
> > sys 0m8.380s
> > 
> > This is ~19% slowdown.
> > 
> > 3. I've run bench_pipes benchmark:
> > https://raw.githubusercontent.com/google/sanitizers/master/address-sanitizer/kernel_buildbot/slave/bench_pipes.c
> > as:
> > while true; do time ./bench_pipes 10 10000 1; done
> > 
> > Before:
> > sys 0m5.393s
> > sys 0m6.178s
> > sys 0m5.909s
> > sys 0m6.024s
> > sys 0m5.874s
> > sys 0m5.737s
> > sys 0m5.826s
> > sys 0m5.664s
> > sys 0m5.758s
> > sys 0m5.421s
> > sys 0m5.444s
> > sys 0m5.479s
> > sys 0m5.461s
> > sys 0m5.417s
> > 
> > After:
> > sys 0m8.718s
> > sys 0m8.281s
> > sys 0m8.268s
> > sys 0m8.334s
> > sys 0m8.246s
> > sys 0m8.267s
> > sys 0m8.265s
> > sys 0m8.437s
> > sys 0m8.228s
> > sys 0m8.312s
> > sys 0m8.556s
> > sys 0m8.680s
> > 
> > This is ~52% slowdown.
> > 
> > 
> > This does not look acceptable to me. I would ready to pay for this,
> > say, 10% of performance. But it seems that this can have up to 2-4x
> > slowdown for some workloads.
> 
> I found the reasons of above regression. There are two reasons.
> 
> 1. In my implementation, original shadow to the memory allocated from
> memblock is black shadow so it causes to call kasan_report(). It will
> pass the check since per page shadow would be zero shadow but it
> causes some overhead.
> 
> 2. Memory used by stackdepot is in a similar situation with #1. It
> allocates page and divide it to many objects. Then, use it like as
> object. Although there is "KASAN_SANITIZE_stackdepot.o := n" which try
> to disable sanitizer, there is a function call (memcmp() in
> find_stack()) to other file and sanitizer work for it.
> 
> #1 problem can be fixed but more investigation is needed. I will
> respin the series after fixing it.
> 
> #2 problem also can be fixed. There are two options here. First, uses
> private memcmp() for stackdepot and disable sanitizer for it. I think
> that this is a right approach since it slowdown the performance in all
> KASAN build cases. And, we don't want to sanitize KASAN itself.
> Second, I can provide a function to map the actual shadow manually. It
> will reduce the case calling kasan_report().
> 
> See the attached patch. It implements later approach on #2 problem.
> It would reduce performance regression. I have tested your bench_pipes
> test with it and found that performance is restored. However, there is
> still remaining problem, #1, so I'm not sure that it completely
> restore your regression. Could you check that if possible?
> 
Oops... I missed to attach the patch.

Thanks.

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