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

From: Alexander Potapenko
Date: Fri Mar 11 2016 - 09:50:08 EST

On Fri, Mar 11, 2016 at 12:43 PM, Andrey Ryabinin
<ryabinin.a.a@xxxxxxxxx> wrote:
> On 03/11/2016 02:18 PM, Alexander Potapenko wrote:
>> On Thu, Mar 10, 2016 at 5:58 PM, Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx> wrote:
>>> 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.
>> I've attempted to add __vmalloc(STACK_ALLOC_SIZE, alloc_flags,
>> PAGE_KERNEL) (also tried vmalloc(STACK_ALLOC_SIZE)) instead of
>> page_alloc() and am now getting a crash in
>> kmem_cache_alloc_node_trace() in mm/slab.c, because it doesn't allow
>> the kmem_cache pointer to be NULL (it's dereferenced when calling
>> trace_kmalloc_node()).
>> Steven, do you know if this because of my code violating some contract
>> (e.g. I'm calling vmalloc() too early, when kmalloc_caches[] haven't
>> been initialized),
> Probably. kmem_cache_init() goes before vmalloc_init().
The solution I'm currently testing is to introduce a per-CPU recursion
flag that depot_save_stack() checks and bails out if it's set.
In addition I look at |kmalloc_caches[KMALLOC_SHIFT_HIGH]| and
in_interrupt() to see if vmalloc() is available.
In the case it is not, I fall back to alloc_pages().

Right now (after 20 minutes of running Trinity) vmalloc() has been
called 490 times, alloc_pages() - only 13 times.
I hope it's now much better from the fragmentation point of view.
>> or is this a bug in kmem_cache_alloc_node_trace()
>> itself?

Alexander Potapenko
Software Engineer

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