Re: [PATCH 1/1] kasan: fix livelock in qlist_move_cache

[Zhouyi Zhou <zhouzhouyi@gmail.com>]

Hi,
    Imagine all of the QUARANTINE_BATCHES elements of
global_quarantine array is of size 4MB + 1MB, now a new call
to quarantine_put is invoked, one of the element will be of size 4MB +
1MB + 1MB, so on and on.

On Tue, Nov 28, 2017 at 4:58 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> On Tue, Nov 28, 2017 at 9:33 AM, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
>> Hi,
>>    Please take a look at function quarantine_put, I don't think following
>> code will limit the batch size below quarantine_batch_size. It only advance
>> quarantine_tail after qlist_move_all.
>>
>>                 qlist_move_all(q, &temp);
>>
>>                 spin_lock(&quarantine_lock);
>>                 WRITE_ONCE(quarantine_size, quarantine_size + temp.bytes);
>>                 qlist_move_all(&temp, &global_quarantine[quarantine_tail]);
>>                 if (global_quarantine[quarantine_tail].bytes >=
>>                                 READ_ONCE(quarantine_batch_size)) {
>>                         int new_tail;
>>
>>                         new_tail = quarantine_tail + 1;
>>                         if (new_tail == QUARANTINE_BATCHES)
>>                                 new_tail = 0;
>>                         if (new_tail != quarantine_head)
>>                                 quarantine_tail = new_tail;
>
>
> As far as I see this code can exceed global quarantine batch size by
> at most 1 per-cpu batch. Per-cpu batch is caped at 1MB. So max global
> batch size will be 4MB+1MB. Which does not radically change situation.
>
>
>> On Tue, Nov 28, 2017 at 4:12 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>> On Tue, Nov 28, 2017 at 9:00 AM, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
>>>> Thanks for reviewing
>>>>    My machine has 128G of RAM, and runs many KVM virtual machines.
>>>> libvirtd always
>>>> report "internal error: received hangup / error event on socket" under
>>>> heavy memory load.
>>>>    Then I use perf top -g, qlist_move_cache consumes 100% cpu for
>>>> several minutes.
>>>
>>> For 128GB of RAM, batch size is 4MB. Processing such batch should not
>>> take more than few ms. So I am still struggling  to understand how/why
>>> your change helps and why there are issues in the first place...
>>>
>>>
>>>
>>>> On Tue, Nov 28, 2017 at 3:45 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>>>> On Tue, Nov 28, 2017 at 5:05 AM, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
>>>>>> When there are huge amount of quarantined cache allocates in system,
>>>>>> number of entries in global_quarantine[i] will be great. Meanwhile,
>>>>>> there is no relax in while loop in function qlist_move_cache which
>>>>>> hold quarantine_lock. As a result, some userspace programs for example
>>>>>> libvirt will complain.
>>>>>
>>>>> Hi,
>>>>>
>>>>> The QUARANTINE_BATCHES thing was supposed to fix this problem, see
>>>>> quarantine_remove_cache() function.
>>>>> What is the amount of RAM and number of CPUs in your system?
>>>>> If system has 4GB of RAM, quarantine size is 128MB and that's split
>>>>> into 1024 batches. Batch size is 128KB. Even if that's filled with the
>>>>> smallest objects of size 32, that's only 4K objects. And there is a
>>>>> cond_resched() between processing of every batch.
>>>>> I don't understand why it causes problems in your setup. We use KASAN
>>>>> extremely heavily on hundreds of machines 24x7 and we have not seen
>>>>> any single report from this code...
>>>>>
>>>>>
>>>>>> On Tue, Nov 28, 2017 at 12:04 PM,  <zhouzhouyi@gmail.com> wrote:
>>>>>>> From: Zhouyi Zhou <zhouzhouyi@gmail.com>
>>>>>>>
>>>>>>> This patch fix livelock by conditionally release cpu to let others
>>>>>>> has a chance to run.
>>>>>>>
>>>>>>> Tested on x86_64.
>>>>>>> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
>>>>>>> ---
>>>>>>>  mm/kasan/quarantine.c | 12 +++++++++++-
>>>>>>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
>>>>>>> index 3a8ddf8..33eeff4 100644
>>>>>>> --- a/mm/kasan/quarantine.c
>>>>>>> +++ b/mm/kasan/quarantine.c
>>>>>>> @@ -265,10 +265,13 @@ static void qlist_move_cache(struct qlist_head *from,
>>>>>>>                                    struct kmem_cache *cache)
>>>>>>>  {
>>>>>>>         struct qlist_node *curr;
>>>>>>> +       struct qlist_head tmp_head;
>>>>>>> +       unsigned long flags;
>>>>>>>
>>>>>>>         if (unlikely(qlist_empty(from)))
>>>>>>>                 return;
>>>>>>>
>>>>>>> +       qlist_init(&tmp_head);
>>>>>>>         curr = from->head;
>>>>>>>         qlist_init(from);
>>>>>>>         while (curr) {
>>>>>>> @@ -278,10 +281,17 @@ static void qlist_move_cache(struct qlist_head *from,
>>>>>>>                 if (obj_cache == cache)
>>>>>>>                         qlist_put(to, curr, obj_cache->size);
>>>>>>>                 else
>>>>>>> -                       qlist_put(from, curr, obj_cache->size);
>>>>>>> +                       qlist_put(&tmp_head, curr, obj_cache->size);
>>>>>>>
>>>>>>>                 curr = next;
>>>>>>> +
>>>>>>> +               if (need_resched()) {
>>>>>>> +                       spin_unlock_irqrestore(&quarantine_lock, flags);
>>>>>>> +                       cond_resched();
>>>>>>> +                       spin_lock_irqsave(&quarantine_lock, flags);
>>>>>>> +               }
>>>>>>>         }
>>>>>>> +       qlist_move_all(&tmp_head, from);
>>>>>>>  }
>>>>>>>
>>>>>>>  static void per_cpu_remove_cache(void *arg)
>>>>>>> --
>>>>>>> 2.1.4
>>>>>>>
>>>>>>
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