linux-kernel - Re: [patch 4/4] mm: memcontrol: remove unnecessary PCG_USED pc->mem

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Date:	Wed, 22 Oct 2014 18:11:40 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 4/4] mm: memcontrol: remove unnecessary PCG_USED
 pc->mem_cgroup valid flag

On Mon 20-10-14 11:22:12, Johannes Weiner wrote:
> pc->mem_cgroup had to be left intact after uncharge for the final LRU
> removal, and !PCG_USED indicated whether the page was uncharged.  But
> since 0a31bc97c80c ("mm: memcontrol: rewrite uncharge API") pages are
> uncharged after the final LRU removal.  Uncharge can simply clear the
> pointer and the PCG_USED/PageCgroupUsed sites can test that instead.
> 
> Because this is the last page_cgroup flag, this patch reduces the
> memcg per-page overhead to a single pointer.

Nice. I have an old patch which stuck this flag into page_cgroup pointer
but this is of course much much better!
 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Acked-by: Michal Hocko <mhocko@...e.cz>

Just a nit below

[...]
> @@ -2525,9 +2523,10 @@ struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)

memcg = NULL initialization is not needed now

>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  
>  	pc = lookup_page_cgroup(page);
> -	if (PageCgroupUsed(pc)) {
> -		memcg = pc->mem_cgroup;
> -		if (memcg && !css_tryget_online(&memcg->css))
> +	memcg = pc->mem_cgroup;
> +
> +	if (memcg) {
> +		if (!css_tryget_online(&memcg->css))
>  			memcg = NULL;
>  	} else if (PageSwapCache(page)) {
>  		ent.val = page_private(page);

>  #else
>  static inline void memcg_unregister_all_caches(struct mem_cgroup *memcg)
[...]
-- 
Michal Hocko
SUSE Labs
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