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SubjectRe: [RFC PATCH] mm: thp: grab the lock before manipulation defer list
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@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
> @@ -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

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