Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

[Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>]

On 1/17/20 3:38 PM, Wei Yang wrote:
> If compound is true, this means it is a PMD mapped THP. Which implies
> the page is not linked to any defer list. So the first code chunk will
> not be executed.
>
> Also with this reason, it would not be proper to add this page to a
> defer list. So the second code chunk is not correct.
>
> Based on this, we should remove the defer list related code.
>
> Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
>
> Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
> Suggested-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>    [5.4+]
>
> ---
> v4:
>    * finally we identified the related code is not necessary and not
>      correct, just remove it
>    * thanks to Kirill T first spot some problem

Thanks for debugging and figuring this out. Acked-by: Yang Shi 
<yang.shi@xxxxxxxxxxxxxxxxx>

> v3:
>    * remove all review/ack tag since rewrite the changelog
>    * use deferred_split_huge_page as the example of race
>    * add cc stable 5.4+ tag as suggested by David Rientjes
>
> 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, 18 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6c83cf4ed970..27c231bf4565 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5340,14 +5340,6 @@ static int mem_cgroup_move_account(struct page *page,
>   		__mod_lruvec_state(to_vec, NR_WRITEBACK, nr_pages);
>   	}
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	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);
> -	}
> -#endif
>   	/*
>   	 * It is safe to change page->mem_cgroup here because the page
>   	 * is referenced, charged, and isolated - we can't race with
> @@ -5357,16 +5349,6 @@ static int mem_cgroup_move_account(struct page *page,
>   	/* caller should have done css_get */
>   	page->mem_cgroup = to;
>   
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	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);
> -	}
> -#endif
> -
>   	spin_unlock_irqrestore(&from->move_lock, flags);
>   
>   	ret = 0;