Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list

[Wei Yang <richardw.yang@linux.intel.com>]

On Mon, Jan 06, 2020 at 08:18:34AM -0800, Alexander Duyck wrote:
>On Fri, Jan 3, 2020 at 6:34 AM Wei Yang <richardw.yang@linux.intel.com> wrote:
>>
>> As all the other places, we grab the lock before manipulate the defer list.
>> Current implementation may face a race condition.
>>
>> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
>>
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>
>> ---
>> I notice the difference during code reading and just confused about the
>> difference. No specific test is done since limited knowledge about cgroup.
>>
>> Maybe I miss something important?
>> ---
>>  mm/memcontrol.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index bc01423277c5..62b7ec34ef1a 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -5368,12 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
>>         }
>>
>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +       spin_lock(&from->deferred_split_queue.split_queue_lock);
>>         if (compound && !list_empty(page_deferred_list(page))) {
>> -               spin_lock(&from->deferred_split_queue.split_queue_lock);
>>                 list_del_init(page_deferred_list(page));
>>                 from->deferred_split_queue.split_queue_len--;
>> -               spin_unlock(&from->deferred_split_queue.split_queue_lock);
>>         }
>> +       spin_unlock(&from->deferred_split_queue.split_queue_lock);
>>  #endif
>>         /*
>>          * It is safe to change page->mem_cgroup here because the page
>
>So I suspect the lock placement has to do with the compound boolean
>value passed to the function.
>

Hey, Alexander

Thanks for your comment.

>One thing you might want to do is pull the "if (compound)" check out
>and place it outside of the spinlock check. It would then simplify
>this signficantly so it is something like
>if (compound) {
>  spin_lock();
>  list = page_deferred_list(page);
>  if (!list_empty(list)) {
>    list_del_init(list);
>    from->..split_queue_len--;
>  }
>  spin_unlock();
>}
>
>Same for the block below. I would pull the check for compound outside
>of the spinlock call since it is a value that shouldn't change and
>would eliminate an unnecessary lock in the non-compound case.

This is reasonable, if no objection from others, I would change this in v2.


-- 
Wei Yang
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