Re: [PATCH 2/2] mm: memcontrol: drop unnecessary lru locking from mem_cgroup_migrate()

[Michal Hocko]

On Thu 04-02-16 15:07:47, Johannes Weiner wrote:
> Migration accounting in the memory controller used to have to handle
> both oldpage and newpage being on the LRU already; fuse's page cache
> replacement used to pass a recycled newpage that had been uncharged
> but not freed and removed from the LRU, and the memcg migration code
> used to uncharge oldpage to "pass on" the existing charge to newpage.
> 
> Nowadays, pages are no longer uncharged when truncated from the page
> cache, but rather only at free time, so if a LRU page is recycled in
> page cache replacement it'll also still be charged. And we bail out of
> the charge transfer altogether in that case. Tell commit_charge() that
> we know newpage is not on the LRU, to avoid taking the zone->lru_lock
> unnecessarily from the migration path.
> 
> But also, oldpage is no longer uncharged inside migration. We only use
> oldpage for its page->mem_cgroup and page size, so we don't care about
> its LRU state anymore either. Remove any mention from the kernel doc.
> 
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/memcontrol.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3e4199830456..42882c1e7fce 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5489,7 +5489,6 @@ void mem_cgroup_uncharge_list(struct list_head *page_list)
>   * be uncharged upon free.
>   *
>   * Both pages must be locked, @newpage->mapping must be set up.
> - * Either or both pages might be on the LRU already.
>   */
>  void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  {
> @@ -5524,7 +5523,7 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage)
>  		page_counter_charge(&memcg->memsw, nr_pages);
>  	css_get_many(&memcg->css, nr_pages);
>  
> -	commit_charge(newpage, memcg, true);
> +	commit_charge(newpage, memcg, false);
>  
>  	local_irq_disable();
>  	mem_cgroup_charge_statistics(memcg, newpage, compound, nr_pages);
> -- 
> 2.7.0

-- 
Michal Hocko
SUSE Labs