Re: [PATCH v7 5/7] mm, kasan: Stackdepot implementation. Enable stackdepot for SLAB

[Alexander Potapenko <glider@google.com>]

On Tue, Mar 15, 2016 at 1:22 PM, Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:
> 2016-03-15 12:27 GMT+03:00 Alexander Potapenko <glider@google.com>:
>> On Mon, Mar 14, 2016 at 5:56 PM, Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:
>>> 2016-03-14 13:43 GMT+03:00 Alexander Potapenko <glider@google.com>:
>>>
>>>> +
>>>> +       rec = this_cpu_ptr(&depot_recursion);
>>>> +       /* Don't store the stack if we've been called recursively. */
>>>> +       if (unlikely(*rec))
>>>> +               goto fast_exit;
>>>> +       *rec = true;
>>>
>>>
>>> This just can't work. As long as preemption enabled, task could
>>> migrate on another cpu anytime.
>> Ah, you're right.
>> Do you think disabling preemption around memory allocation is an option here?
>
> It's definitely not an option. Flag on current doesn't have any
> disadvantage over per-cpu approach
> and it doesn't require preemption safe context.
> However, making the allocation in a separate context would be a better
> way to eliminate recursion.
> i.e. instead of allocating memory depot_save_stack() kicks a work
> which allocates memory.

For the record, I've removed the vmalloc code and reinstated
alloc_pages(), so that there's no more recursion.
Making the allocation in a separate worker will remove the recursion,
but may complicate the synchronization.
I'd refrain from that since we don't have problems with recursion right now.
>
>>> You could use per-task flag, although it's possible to miss some
>>> in-irq stacktraces:
>>>
>>> depot_save_stack()
>>>     if (current->stackdeport_recursion)
>>>           goto fast_exit;
>>>     current->stackdepot_recursion++
>>>     <IRQ>
>>>            ....
>>>            depot_save_stack()
>>>                  if (current->stackdeport_recursion)
>>>                       goto fast_exit;
>>>
>>>
>>>
>>>> +       if (unlikely(!smp_load_acquire(&next_slab_inited))) {
>>>> +               /* Zero out zone modifiers, as we don't have specific zone
>>>> +                * requirements. Keep the flags related to allocation in atomic
>>>> +                * contexts and I/O.
>>>> +                */
>>>> +               alloc_flags &= ~GFP_ZONEMASK;
>>>> +               alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
>>>> +               /* When possible, allocate using vmalloc() to reduce physical
>>>> +                * address space fragmentation. vmalloc() doesn't work if
>>>> +                * kmalloc caches haven't been initialized or if it's being
>>>> +                * called from an interrupt handler.
>>>> +                */
>>>> +               if (kmalloc_caches[KMALLOC_SHIFT_HIGH] && !in_interrupt()) {
>>>
>>> This is clearly a wrong way to check whether is slab available or not.
>> Well, I don't think either vmalloc() or kmalloc() provide any
>> interface to check if they are available.
>>
>>> Besides you need to check
>>> vmalloc() for availability, not slab.
>> The problem was in kmalloc caches being unavailable, although I can
>> imagine other problems could have arose.
>> Perhaps we can drill a hole to get the value of vmap_initialized?
>>> Given that STAC_ALLOC_ORDER is 2 now, I think it should be fine to use
>>> alloc_pages() all the time.
>>> Or fix condition, up to you.
>> Ok, I'm going to drop vmalloc() for now, we can always implement this later.
>> Note that this also removes the necessity to check for recursion.
>>>> +                       prealloc = __vmalloc(
>>>> +                               STACK_ALLOC_SIZE, alloc_flags, PAGE_KERNEL);
>>>> +               } else {
>>>> +                       page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>>>> +                       if (page)
>>>> +                               prealloc = page_address(page);
>>>> +               }
>>>> +       }
>>>> +
>>
>>
>>
>> --
>> Alexander Potapenko
>> Software Engineer
>>
>> Google Germany GmbH
>> Erika-Mann-Straße, 33
>> 80636 München
>>
>> Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle
>> Registergericht und -nummer: Hamburg, HRB 86891
>> Sitz der Gesellschaft: Hamburg



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Matthew Scott Sucherman, Paul Terence Manicle
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg

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