linux-kernel - Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two atomic ops in zs_pool_dec

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Date:   Thu, 1 Jul 2021 10:43:45 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>, <ngupta@...are.org>,
        <senozhatsky@...omium.org>, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [Phishing Risk] [External] [PATCH 2/3] mm/zsmalloc.c: combine two
 atomic ops in zs_pool_dec_isolated()

On 2021/6/25 18:40, Muchun Song wrote:
> On Fri, Jun 25, 2021 at 5:32 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>
>> On 2021/6/25 16:46, Miaohe Lin wrote:
>>> On 2021/6/25 15:29, Muchun Song wrote:
>>>> On Fri, Jun 25, 2021 at 2:32 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>>>>
>>>>> On 2021/6/25 13:01, Muchun Song wrote:
>>>>>> On Thu, Jun 24, 2021 at 8:40 PM Miaohe Lin <linmiaohe@...wei.com> wrote:
>>>>>>>
>>>>>>> atomic_long_dec_and_test() is equivalent to atomic_long_dec() and
>>>>>>> atomic_long_read() == 0. Use it to make code more succinct.
>>>>>>
>>>>>> Actually, they are not equal. atomic_long_dec_and_test implies a
>>>>>> full memory barrier around it but atomic_long_dec and atomic_long_read
>>>>>> don't.
>>>>>>
>>>>>
>>>>> Many thanks for comment. They are indeed not completely equal as you said.
>>>>> What I mean is they can do the same things we want in this specified context.
>>>>> Thanks again.
>>>>
>>>> I don't think so. Using individual operations can eliminate memory barriers.
>>>> We will pay for the barrier if we use atomic_long_dec_and_test here.
>>>
>>> The combination of atomic_long_dec and atomic_long_read usecase is rare and looks somehow
>>> weird. I think it's worth to do this with the cost of barrier.
>>>
>>
>> It seems there is race between zs_pool_dec_isolated and zs_unregister_migration if pool->destroying
>> is reordered before the atomic_long_dec and atomic_long_read ops. So this memory barrier is necessary:
>>
>> zs_pool_dec_isolated                            zs_unregister_migration
>>   pool->destroying != true
>>                                                   pool->destroying = true;
>>                                                   smp_mb();
>>                                                   wait_for_isolated_drain
>>                                                     wait_event with atomic_long_read(&pool->isolated_pages) != 0
>>   atomic_long_dec(&pool->isolated_pages);
>>   atomic_long_read(&pool->isolated_pages) == 0
> 
> I am not familiar with zsmalloc. So I do not know whether the race
> that you mentioned above exists. But If it exists, the fix also does
> not make sense to me. If there should be inserted a smp_mb between
> atomic_long_dec and atomic_long_read, you should insert
> smp_mb__after_atomic instead of using atomic_long_dec_and_test.
> Because smp_mb__after_atomic can be optimized on certain architecture
> (e.g. x86_64).
> 

Sorry for the delay.

I think there is two options:

atomic_long_dec(&pool->isolated_pages);
smp_mb__after_atomic
atomic_long_read(&pool->isolated_pages) == 0

We have two atomic ops with one smp_mb.

vs

atomic_long_dec_and_test while implies __smp_mb__before_atomic + atomic_long_ops + smp_mb__after_atomic.

We have one atomic ops with two smp_mb.

I think either one works but prefer later one. What do you think?

Thanks.

> Thanks.
> 
>>
>> Thus wake_up_all is missed.
>> And the comment in zs_pool_dec_isolated() said:
>> /*
>>  * There's no possibility of racing, since wait_for_isolated_drain()
>>  * checks the isolated count under &class->lock after enqueuing
>>  * on migration_wait.
>>  */
>>
>> But I found &class->lock is indeed not acquired for wait_for_isolated_drain(). So I think the above race
>> is possible. Does this make senses for you ?
>> Thanks.
>>
>>>>
>>>>>
>>>>>> That RMW operations that have a return value is equal to the following.
>>>>>>
>>>>>> smp_mb__before_atomic()
>>>>>> non-RMW operations or RMW operations that have no return value
>>>>>> smp_mb__after_atomic()
>>>>>>
>>>>>> Thanks.
>>>>>>
>>>>>>>
>>>>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>>>>>> ---
>>>>>>>  mm/zsmalloc.c | 3 +--
>>>>>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
>>>>>>> index 1476289b619f..0b4b23740d78 100644
>>>>>>> --- a/mm/zsmalloc.c
>>>>>>> +++ b/mm/zsmalloc.c
>>>>>>> @@ -1828,13 +1828,12 @@ static void putback_zspage_deferred(struct zs_pool *pool,
>>>>>>>  static inline void zs_pool_dec_isolated(struct zs_pool *pool)
>>>>>>>  {
>>>>>>>         VM_BUG_ON(atomic_long_read(&pool->isolated_pages) <= 0);
>>>>>>> -       atomic_long_dec(&pool->isolated_pages);
>>>>>>>         /*
>>>>>>>          * There's no possibility of racing, since wait_for_isolated_drain()
>>>>>>>          * checks the isolated count under &class->lock after enqueuing
>>>>>>>          * on migration_wait.
>>>>>>>          */
>>>>>>> -       if (atomic_long_read(&pool->isolated_pages) == 0 && pool->destroying)
>>>>>>> +       if (atomic_long_dec_and_test(&pool->isolated_pages) && pool->destroying)
>>>>>>>                 wake_up_all(&pool->migration_wait);
>>>>>>>  }
>>>>>>>
>>>>>>> --
>>>>>>> 2.23.0
>>>>>>>
>>>>>> .
>>>>>>
>>>>>
>>>> .
>>>>
>>>
>>
> .
>