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

[Michal Hocko]

On Fri 03-01-20 22:34:07, Wei Yang wrote:
> As all the other places, we grab the lock before manipulate the defer list.
> Current implementation may face a race condition.

Please always make sure to describe the effect of the change. Why a racy
list_empty check matters?

> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> 
> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> 
> ---
> 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
> @@ -5385,13 +5385,13 @@ static int mem_cgroup_move_account(struct page *page,
>  	page->mem_cgroup = to;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +	spin_lock(&to->deferred_split_queue.split_queue_lock);
>  	if (compound && list_empty(page_deferred_list(page))) {
> -		spin_lock(&to->deferred_split_queue.split_queue_lock);
>  		list_add_tail(page_deferred_list(page),
>  			      &to->deferred_split_queue.split_queue);
>  		to->deferred_split_queue.split_queue_len++;
> -		spin_unlock(&to->deferred_split_queue.split_queue_lock);
>  	}
> +	spin_unlock(&to->deferred_split_queue.split_queue_lock);
>  #endif
>  
>  	spin_unlock_irqrestore(&from->move_lock, flags);
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs