Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

[Kirill A. Shutemov]

On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote:
> As all the other places, we grab the lock before manipulate the defer list.
> Current implementation may face a race condition.
> 
> For example, the potential race would be:
> 
>     CPU1                      CPU2
>     mem_cgroup_move_account   split_huge_page_to_list
>       !list_empty
>                                 lock
>                                 !list_empty
>                                 list_del
>                                 unlock
>       lock
>       # !list_empty might not hold anymore
>       list_del_init
>       unlock

I don't think this particular race is possible. Both parties take page
lock before messing with deferred queue, but anytway:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

> 
> When this sequence happens, the list_del_init() in
> mem_cgroup_move_account() would crash if CONFIG_DEBUG_LIST since the
> page is already been removed by list_del in split_huge_page_to_list().
> 
> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> 
> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> 
> ---
> v2:
>   * move check on compound outside suggested by Alexander
>   * an example of the race condition, suggested by Michal
> ---
>  mm/memcontrol.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bc01423277c5..1492eefe4f3c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5368,10 +5368,12 @@ static int mem_cgroup_move_account(struct page *page,
>  	}
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && !list_empty(page_deferred_list(page))) {
> +	if (compound) {
>  		spin_lock(&from->deferred_split_queue.split_queue_lock);
> -		list_del_init(page_deferred_list(page));
> -		from->deferred_split_queue.split_queue_len--;
> +		if (!list_empty(page_deferred_list(page))) {
> +			list_del_init(page_deferred_list(page));
> +			from->deferred_split_queue.split_queue_len--;
> +		}
>  		spin_unlock(&from->deferred_split_queue.split_queue_lock);
>  	}
>  #endif
> @@ -5385,11 +5387,13 @@ static int mem_cgroup_move_account(struct page *page,
>  	page->mem_cgroup = to;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (compound && list_empty(page_deferred_list(page))) {
> +	if (compound) {
>  		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++;
> +		if (list_empty(page_deferred_list(page))) {
> +			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);
>  	}
>  #endif
> -- 
> 2.17.1
> 
> 

-- 
 Kirill A. Shutemov