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Date:   Thu, 26 May 2022 16:17:27 -0400
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Subject: Re: [PATCH v4 03/11] mm: memcontrol: make lruvec lock safe when LRU
 pages are reparented
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To:     Muchun Song <songmuchun@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Cc:     mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, duanxiongchun@bytedance.com
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From:   Waiman Long <longman@redhat.com>
In-Reply-To: <Yo5NdncOsqL0xP8Q@FVFYT0MHHV2J.googleapis.com>
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On 5/25/22 11:38, Muchun Song wrote:
> On Wed, May 25, 2022 at 10:48:54AM -0400, Johannes Weiner wrote:
>> On Wed, May 25, 2022 at 09:03:59PM +0800, Muchun Song wrote:
>>> On Wed, May 25, 2022 at 08:30:15AM -0400, Johannes Weiner wrote:
>>>> On Wed, May 25, 2022 at 05:53:30PM +0800, Muchun Song wrote:
>>>>> On Tue, May 24, 2022 at 03:27:20PM -0400, Johannes Weiner wrote:
>>>>>> On Tue, May 24, 2022 at 02:05:43PM +0800, Muchun Song wrote:
>>>>>>> @@ -1230,10 +1213,23 @@ void lruvec_memcg_debug(struct lruvec *lruvec, struct folio *folio)
>>>>>>>    */
>>>>>>>   struct lruvec *folio_lruvec_lock(struct folio *folio)
>>>>>>>   {
>>>>>>> -	struct lruvec *lruvec = folio_lruvec(folio);
>>>>>>> +	struct lruvec *lruvec;
>>>>>>>   
>>>>>>> +	rcu_read_lock();
>>>>>>> +retry:
>>>>>>> +	lruvec = folio_lruvec(folio);
>>>>>>>   	spin_lock(&lruvec->lru_lock);
>>>>>>> -	lruvec_memcg_debug(lruvec, folio);
>>>>>>> +
>>>>>>> +	if (unlikely(lruvec_memcg(lruvec) != folio_memcg(folio))) {
>>>>>>> +		spin_unlock(&lruvec->lru_lock);
>>>>>>> +		goto retry;
>>>>>>> +	}
>>>>>>> +
>>>>>>> +	/*
>>>>>>> +	 * Preemption is disabled in the internal of spin_lock, which can serve
>>>>>>> +	 * as RCU read-side critical sections.
>>>>>>> +	 */
>>>>>>> +	rcu_read_unlock();
>>>>>> The code looks right to me, but I don't understand the comment: why do
>>>>>> we care that the rcu read-side continues? With the lru_lock held,
>>>>>> reparenting is on hold and the lruvec cannot be rcu-freed anyway, no?
>>>>>>
>>>>> Right. We could hold rcu read lock until end of reparting.  So you mean
>>>>> we do rcu_read_unlock in folio_lruvec_lock()?
>>>> The comment seems to suggest that disabling preemption is what keeps
>>>> the lruvec alive. But it's the lru_lock that keeps it alive. The
>>>> cgroup destruction path tries to take the lru_lock long before it even
>>>> gets to synchronize_rcu(). Once you hold the lru_lock, having an
>>>> implied read-side critical section as well doesn't seem to matter.
>>>>
>>> Well, I thought that spinlocks have implicit read-side critical sections
>>> because it disables preemption (I learned from the comments above
>>> synchronize_rcu() that says interrupts, preemption, or softirqs have been
>>> disabled also serve as RCU read-side critical sections).  So I have a
>>> question: is it still true in a PREEMPT_RT kernel (I am not familiar with
>>> this)?
>> Yes, but you're missing my point.
>>
>>>> Should the comment be deleted?
>>> I think we could remove the comments. If the above question is false, seems
>>> like we should continue holding rcu read lock.
>> It's true.
>>
> Thanks for your answer.
>
>> But assume it's false for a second. Why would you need to continue
>> holding it? What would it protect? The lruvec would be pinned by the
>> spinlock even if it DIDN'T imply an RCU lock, right?
>>
>> So I don't understand the point of the comment. If the implied RCU
>> lock is protecting something not covered by the bare spinlock itself,
>> it should be added to the comment. Otherwise, the comment should go.
>>
> Got it. Thanks for your nice explanation. I'll remove
> the comment here.

Note that there is a similar comment in patch 6 which may have to be 
removed as well.

Cheers,
Longman