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

[Alexander Duyck <alexander.duyck@gmail.com>]

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.

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.

> @@ -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);
> --