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
