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

[Alexander Potapenko]

On Mon, Mar 14, 2016 at 5:56 PM, Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx> wrote:
> 2016-03-14 13:43 GMT+03:00 Alexander Potapenko <glider@xxxxxxxxxx>:
>
>> +
>> +       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?
> 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

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