linux-kernel - Re: [PATCH 1/1] kasan: fix livelock in qlist_move

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Date:   Tue, 28 Nov 2017 08:45:54 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] kasan: fix livelock in qlist_move_cache

On Tue, Nov 28, 2017 at 5:05 AM, Zhouyi Zhou <zhouzhouyi@...il.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@...il.com> wrote:
>> From: Zhouyi Zhou <zhouzhouyi@...il.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@...il.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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