Re: [External] Re: [PATCH 5/5] mm, memcg: always call __mod_node_page_state() with preempt disabled

[Miaohe Lin]

On 2021/7/30 10:33, Muchun Song wrote:
> On Fri, Jul 30, 2021 at 9:52 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> On 2021/7/29 22:39, Shakeel Butt wrote:
>>> On Thu, Jul 29, 2021 at 5:58 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>>>
>>>> We should always ensure __mod_node_page_state() is called with preempt
>>>> disabled or percpu ops may manipulate the wrong cpu when preempt happened.
>>>>
>>>> Fixes: b4e0b68fbd9d ("mm: memcontrol: use obj_cgroup APIs to charge kmem pages")
>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>> ---
>>>>  mm/memcontrol.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>>> index 70a32174e7c4..616d1a72ece3 100644
>>>> --- a/mm/memcontrol.c
>>>> +++ b/mm/memcontrol.c
>>>> @@ -697,8 +697,8 @@ void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
>>>>         memcg = page_memcg(head);
>>>>         /* Untracked pages have no memcg, no lruvec. Update only the node */
>>>>         if (!memcg) {
>>>> -               rcu_read_unlock();
>>>>                 __mod_node_page_state(pgdat, idx, val);
>>>> +               rcu_read_unlock();
>>>
>>> This rcu is for page_memcg. The preemption and interrupts are disabled
>>> across __mod_lruvec_page_state().
>>>
>>
>> I thought it's used to protect __mod_node_page_state(). Looks somewhat confusing for me.
>> Many thanks for pointing this out!
> 
> Hi Miaohe,
> 
> git show b4e0b68fbd9d can help you find out why we add
> the rcu read lock around it.

Thanks for your tip. That's my overlook when I checked this commit. I should have looked at this
more closely. :(

> 
> Thanks.
> 
>>
>>>>                 return;
>>>>         }
>>>>
>>>> --
>>>> 2.23.0
>>>>
>>> .
>>>
>>
> .
>