Re: [patch 04/11] mm: memcg: push down PageSwapCache check into uncharge entry functions

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/07/05 9:44), Johannes Weiner wrote:
> Not all uncharge paths need to check if the page is swapcache, some of
> them can know for sure.
> 
> Push down the check into all callsites of uncharge_common() so that
> the patch that removes some of them is more obvious.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---

some nitpick.

>   mm/memcontrol.c |   18 ++++++++++++------
>   1 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 4ea19c6..a3bf414 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2920,8 +2920,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype,
>   	if (mem_cgroup_disabled())
>   		return NULL;
>   
> -	if (PageSwapCache(page))
> -		return NULL;
> +	VM_BUG_ON(PageSwapCache(page));
>   
>   	if (PageTransHuge(page)) {
>   		nr_pages <<= compound_order(page);
> @@ -3018,6 +3017,8 @@ void mem_cgroup_uncharge_page(struct page *page)
>   	if (page_mapped(page))
>   		return;
>   	VM_BUG_ON(page->mapping && !PageAnon(page));
> +	if (PageSwapCache(page))
> +		return;
>   	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_ANON, false);
>   }
>   
> @@ -3025,6 +3026,8 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
>   {
>   	VM_BUG_ON(page_mapped(page));
>   	VM_BUG_ON(page->mapping);
> +	if (PageSwapCache(page))
> +		return;
>   	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE, false);
>   }
>   
> @@ -3089,6 +3092,8 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
>   	if (!swapout) /* this was a swap cache but the swap is unused ! */
>   		ctype = MEM_CGROUP_CHARGE_TYPE_DROP;
>   
> +	if (PageSwapCache(page))
> +		return;
>   	memcg = __mem_cgroup_uncharge_common(page, ctype, false);
>   
>   	/*
> @@ -3278,10 +3283,11 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
>   		unused = oldpage;
>   	}
>   	anon = PageAnon(used);
> -	__mem_cgroup_uncharge_common(unused,
> -		anon ? MEM_CGROUP_CHARGE_TYPE_ANON
> -		     : MEM_CGROUP_CHARGE_TYPE_CACHE,
> -		true);
> +	if (!PageSwapCache(page))
> +		__mem_cgroup_uncharge_common(unused,
> +					     anon ? MEM_CGROUP_CHARGE_TYPE_ANON
> +					     : MEM_CGROUP_CHARGE_TYPE_CACHE,
> +					     true);

!PageSwapCache(unused) ?

But I think unused page's PG_swapcache is always dropped. So, the check is
not necessary.

Thanks,
-Kame