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

Konstantin Khlebnikov
2020-01-10T08:49:15Z

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@linux.alibaba.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> 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,
>