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

[Zhouyi Zhou]

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;

On Tue, Nov 28, 2017 at 4:12 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Tue, Nov 28, 2017 at 9:00 AM, Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>> On Tue, Nov 28, 2017 at 5:05 AM, Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> 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@xxxxxxxxx> wrote:
>>>>> From: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxx>
>>>>> ---
>>>>>  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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