Re: [PATCH v7 02/10] mm/memcg: fold lru_lock in lock_page_lru

[Konstantin Khlebnikov]

On 25/12/2019 12.04, Alex Shi wrote:
>  From the commit_charge's explanations and mem_cgroup_commit_charge
> comments, as well as call path when lrucare is ture, The lru_lock is
> just to guard the task migration(which would be lead to move_account)
> So it isn't needed when !PageLRU, and better be fold into PageLRU to
> reduce lock contentions.
>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>   mm/memcontrol.c | 9 ++++-----
>   1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c5b5f74cfd4d..0ad10caabc3d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2572,12 +2572,11 @@ static void cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>   
>   static void lock_page_lru(struct page *page, int *isolated)
>   {
> -	pg_data_t *pgdat = page_pgdat(page);
> -
> -	spin_lock_irq(&pgdat->lru_lock);
>   	if (PageLRU(page)) {
> +		pg_data_t *pgdat = page_pgdat(page);
>   		struct lruvec *lruvec;
>   
> +		spin_lock_irq(&pgdat->lru_lock);

That's wrong. Here PageLRU must be checked again under lru_lock.


Also I don't like these functions:
- called lock/unlock but actually also isolates
- used just once
- pgdat evaluated twice

>   		lruvec = mem_cgroup_page_lruvec(page, pgdat);
>   		ClearPageLRU(page);
>   		del_page_from_lru_list(page, lruvec, page_lru(page));
> @@ -2588,17 +2587,17 @@ static void lock_page_lru(struct page *page, int *isolated)
>   
>   static void unlock_page_lru(struct page *page, int isolated)
>   {
> -	pg_data_t *pgdat = page_pgdat(page);
>   
>   	if (isolated) {
> +		pg_data_t *pgdat = page_pgdat(page);
>   		struct lruvec *lruvec;
>   
>   		lruvec = mem_cgroup_page_lruvec(page, pgdat);
>   		VM_BUG_ON_PAGE(PageLRU(page), page);
>   		SetPageLRU(page);
>   		add_page_to_lru_list(page, lruvec, page_lru(page));
> +		spin_unlock_irq(&pgdat->lru_lock);
>   	}
> -	spin_unlock_irq(&pgdat->lru_lock);
>   }
>   
>   static void commit_charge(struct page *page, struct mem_cgroup *memcg,