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

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Message-ID: <20120709073614.GB1779@cmpxchg.org>
Date:	Mon, 9 Jul 2012 09:36:14 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 04/11] mm: memcg: push down PageSwapCache check into
 uncharge entry functions

On Mon, Jul 09, 2012 at 11:42:12AM +0900, Kamezawa Hiroyuki wrote:
> (2012/07/05 9:44), Johannes Weiner wrote:
> > @@ -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) ?

Argh, right.

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

Oh, this is intentional: the check was in __mem_cgroup_uncharge_common
before, which means it applied to this entry point as well.  This is
supposed to be a mechanical change that does not change any logic.
The check is then removed in the next patch.

---
Subject: mm: memcg: push down PageSwapCache check into uncharge entry functions fix

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a3bf414..f4ff18a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3283,7 +3283,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 		unused = oldpage;
 	}
 	anon = PageAnon(used);
-	if (!PageSwapCache(page))
+	if (!PageSwapCache(unused))
 		__mem_cgroup_uncharge_common(unused,
 					     anon ? MEM_CGROUP_CHARGE_TYPE_ANON
 					     : MEM_CGROUP_CHARGE_TYPE_CACHE,

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