Re: [PATCH v4 5/7] mm, kasan: Stackdepot implementation. Enable stackdepot for SLAB

From: Andrey Ryabinin
Date: Thu Mar 10 2016 - 12:00:04 EST

2016-03-08 14:42 GMT+03:00 Alexander Potapenko <glider@xxxxxxxxxx>:
> On Tue, Mar 1, 2016 at 12:57 PM, Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx> wrote:
>>>>> + page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>>>> STACK_ALLOC_ORDER = 4 - that's a lot. Do you really need that much?
>>> Part of the issue the atomic context above. When we can't allocate
>>> memory we still want to save the stack trace. When we have less than
>>> STACK_ALLOC_ORDER memory, we try to preallocate another
>>> STACK_ALLOC_ORDER in advance. So in the worst case, we have
>>> STACK_ALLOC_ORDER memory and that should be enough to handle all
>>> kmalloc/kfree in the atomic context. 1 page does not look enough. I
>>> think Alex did some measuring of the failure race (when we are out of
>>> memory and can't allocate more).
>> A lot of 4-order pages will lead to high fragmentation. You don't need physically contiguous memory here,
>> so try to use vmalloc(). It is slower, but fragmentation won't be problem.
> I've tried using vmalloc(), but turned out it's calling KASAN hooks
> again. Dealing with reentrancy in this case sounds like an overkill.

We'll have to deal with recursion eventually. Using stackdepot for
page owner will cause recursion.

> Given that we only require 9 Mb most of the time, is allocating
> physical pages still a problem?

This is not about size, this about fragmentation. vmalloc allows to
utilize available low-order pages,
hence reduce the fragmentation.

>> And one more thing. Take a look at mempool, because it's generally used to solve the problem you have here
>> (guaranteed allocation in atomic context).
> As far as I understood the docs, mempools have a drawback of
> allocating too much memory which won't be available for any other use.

As far as I understood your code, it has a drawback of
allocating too much memory which won't be available for any other use ;)

However, now I think that mempool doesn't fit here. We never free
memory => never return it to pool.
And this will cause 5sec delays between allocation retries in mempool_alloc().

> O'Reily's "Linux Device Drivers" even suggests not using mempools in
> any case when it's easier to deal with allocation failures (that
> advice is for device drivers, not sure if that stands for other
> subsystems though).
> --
> Alexander Potapenko
> Software Engineer
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