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

[Dmitry Vyukov <dvyukov@google.com>]

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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