Re: [patch 2/4] mm: memcontrol: remove unnecessary PCG_MEMSW memory+swap charge flag

[Michal Hocko <mhocko@suse.cz>]

On Mon 20-10-14 11:22:10, Johannes Weiner wrote:
> Now that mem_cgroup_swapout() fully uncharges the page, every page
> that is still in use when reaching mem_cgroup_uncharge() is known to
> carry both the memory and the memory+swap charge.  Simplify the
> uncharge path and remove the PCG_MEMSW page flag accordingly.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Looks good
Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
>  include/linux/page_cgroup.h |  1 -
>  mm/memcontrol.c             | 34 ++++++++++++----------------------
>  2 files changed, 12 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
> index 5c831f1eca79..da62ee2be28b 100644
> --- a/include/linux/page_cgroup.h
> +++ b/include/linux/page_cgroup.h
> @@ -5,7 +5,6 @@ enum {
>  	/* flags for mem_cgroup */
>  	PCG_USED = 0x01,	/* This page is charged to a memcg */
>  	PCG_MEM = 0x02,		/* This page holds a memory charge */
> -	PCG_MEMSW = 0x04,	/* This page holds a memory+swap charge */
>  };
>  
>  struct pglist_data;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7709f17347f3..9bab35fc3e9e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2606,7 +2606,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg,
>  	 *   have the page locked
>  	 */
>  	pc->mem_cgroup = memcg;
> -	pc->flags = PCG_USED | PCG_MEM | (do_swap_account ? PCG_MEMSW : 0);
> +	pc->flags = PCG_USED | PCG_MEM;
>  
>  	if (lrucare)
>  		unlock_page_lru(page, isolated);
> @@ -5815,7 +5815,6 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
>  	if (!PageCgroupUsed(pc))
>  		return;
>  
> -	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEMSW), page);
>  	memcg = pc->mem_cgroup;
>  
>  	oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
> @@ -6010,17 +6009,16 @@ void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg)
>  }
>  
>  static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
> -			   unsigned long nr_mem, unsigned long nr_memsw,
>  			   unsigned long nr_anon, unsigned long nr_file,
>  			   unsigned long nr_huge, struct page *dummy_page)
>  {
> +	unsigned long nr_pages = nr_anon + nr_file;
>  	unsigned long flags;
>  
>  	if (!mem_cgroup_is_root(memcg)) {
> -		if (nr_mem)
> -			page_counter_uncharge(&memcg->memory, nr_mem);
> -		if (nr_memsw)
> -			page_counter_uncharge(&memcg->memsw, nr_memsw);
> +		page_counter_uncharge(&memcg->memory, nr_pages);
> +		if (do_swap_account)
> +			page_counter_uncharge(&memcg->memsw, nr_pages);
>  		memcg_oom_recover(memcg);
>  	}
>  
> @@ -6029,23 +6027,21 @@ static void uncharge_batch(struct mem_cgroup *memcg, unsigned long pgpgout,
>  	__this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_CACHE], nr_file);
>  	__this_cpu_sub(memcg->stat->count[MEM_CGROUP_STAT_RSS_HUGE], nr_huge);
>  	__this_cpu_add(memcg->stat->events[MEM_CGROUP_EVENTS_PGPGOUT], pgpgout);
> -	__this_cpu_add(memcg->stat->nr_page_events, nr_anon + nr_file);
> +	__this_cpu_add(memcg->stat->nr_page_events, nr_pages);
>  	memcg_check_events(memcg, dummy_page);
>  	local_irq_restore(flags);
>  
>  	if (!mem_cgroup_is_root(memcg))
> -		css_put_many(&memcg->css, max(nr_mem, nr_memsw));
> +		css_put_many(&memcg->css, nr_pages);
>  }
>  
>  static void uncharge_list(struct list_head *page_list)
>  {
>  	struct mem_cgroup *memcg = NULL;
> -	unsigned long nr_memsw = 0;
>  	unsigned long nr_anon = 0;
>  	unsigned long nr_file = 0;
>  	unsigned long nr_huge = 0;
>  	unsigned long pgpgout = 0;
> -	unsigned long nr_mem = 0;
>  	struct list_head *next;
>  	struct page *page;
>  
> @@ -6072,10 +6068,9 @@ static void uncharge_list(struct list_head *page_list)
>  
>  		if (memcg != pc->mem_cgroup) {
>  			if (memcg) {
> -				uncharge_batch(memcg, pgpgout, nr_mem, nr_memsw,
> -					       nr_anon, nr_file, nr_huge, page);
> -				pgpgout = nr_mem = nr_memsw = 0;
> -				nr_anon = nr_file = nr_huge = 0;
> +				uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> +					       nr_huge, page);
> +				pgpgout = nr_anon = nr_file = nr_huge = 0;
>  			}
>  			memcg = pc->mem_cgroup;
>  		}
> @@ -6091,18 +6086,14 @@ static void uncharge_list(struct list_head *page_list)
>  		else
>  			nr_file += nr_pages;
>  
> -		if (pc->flags & PCG_MEM)
> -			nr_mem += nr_pages;
> -		if (pc->flags & PCG_MEMSW)
> -			nr_memsw += nr_pages;
>  		pc->flags = 0;
>  
>  		pgpgout++;
>  	} while (next != page_list);
>  
>  	if (memcg)
> -		uncharge_batch(memcg, pgpgout, nr_mem, nr_memsw,
> -			       nr_anon, nr_file, nr_huge, page);
> +		uncharge_batch(memcg, pgpgout, nr_anon, nr_file,
> +			       nr_huge, page);
>  }
>  
>  /**
> @@ -6187,7 +6178,6 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
>  		return;
>  
>  	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
> -	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
>  
>  	if (lrucare)
>  		lock_page_lru(oldpage, &isolated);
> -- 
> 2.1.2
> 

-- 
Michal Hocko
SUSE Labs

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