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

[Andrey Ryabinin <ryabinin.a.a@gmail.com>]

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.


>> 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
>
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