linux-kernel - Re: [PATCH v4] memcg: remove PCG_CACHE page

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Message-ID: <20120124180842.GA18372@tiehlicka.suse.cz>
Date:	Tue, 24 Jan 2012 19:09:47 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH v4] memcg: remove PCG_CACHE page_cgroup flag

On Tue 24-01-12 18:23:08, Michal Hocko wrote:
> On Tue 24-01-12 17:44:49, Johannes Weiner wrote:
> > On Tue, Jan 24, 2012 at 05:01:40PM +0100, Michal Hocko wrote:
> > > On Tue 24-01-12 15:54:11, Johannes Weiner wrote:
> > > > Hold on, I think this patch is still not complete: end_migration()
> > > > directly uses __mem_cgroup_uncharge_common() with the FORCE charge
> > > > type.  This will uncharge all migrated anon pages as cache, when it
> > > > should decide based on PageAnon(used), which is the page where
> > > > ->mapping is intact after migration.
> > > 
> > > You are right, I've missed that one as well. Anyway
> > > MEM_CGROUP_CHARGE_TYPE_FORCE is used only in mem_cgroup_end_migration
> > > these days and it got out of sync with its documentation (used by
> > > force_empty) quite some time ago (f817ed48). What about something like
> > > the following on top of the previous patch?
> > > --- 
> > > Should be foldet into the previous patch with the updated changelog:
> > > 
> > > Mapping of the unused page is not touched during migration (see
> > 
> > used one, not unused.  unused->mapping is globbered during migration.
> 
> Yes, you are right:

Sorry I haven't sent the most recent update. Here we go:
---
Should be folded into the previous patch with the updated changelog:

Mapping of the used page is not touched during migration (see
page_remove_rmap) so we can rely on it and push the correct charge type
down to __mem_cgroup_uncharge_common from end_migration for unused page.
The force flag was misleading was abused for skipping the needless
page_mapped() / PageCgroupMigration() check, as we know the unused page
is no longer mapped and cleared the migration flag just a few lines
up.  But doing the checks is no biggie and it's not worth adding another
flag just to skip them.  But I guess this should be mentioned in the
changelog.
---
 mm/memcontrol.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4d655ee..869744a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3195,6 +3195,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 {
 	struct page *used, *unused;
 	struct page_cgroup *pc;
+	bool anon;
 
 	if (!memcg)
 		return;
@@ -3207,6 +3208,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 		used = newpage;
 		unused = oldpage;
 	}
+
 	/*
 	 * We disallowed uncharge of pages under migration because mapcount
 	 * of the page goes down to zero, temporarly.
@@ -3217,7 +3219,10 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 	ClearPageCgroupMigration(pc);
 	unlock_page_cgroup(pc);
 
-	__mem_cgroup_uncharge_common(unused, MEM_CGROUP_CHARGE_TYPE_FORCE);
+	anon = PageAnon(used);
+	__mem_cgroup_uncharge_common(unused,
+			anon ? MEM_CGROUP_CHARGE_TYPE_MAPPED
+			: MEM_CGROUP_CHARGE_TYPE_CACHE);
 
 	/*
 	 * If a page is a file cache, radix-tree replacement is very atomic
@@ -3227,7 +3232,7 @@ void mem_cgroup_end_migration(struct mem_cgroup *memcg,
 	 * and USED bit check in mem_cgroup_uncharge_page() will do enough
 	 * check. (see prepare_charge() also)
 	 */
-	if (PageAnon(used))
+	if (anon)
 		mem_cgroup_uncharge_page(used);
 	/*
 	 * At migration, we may charge account against cgroup which has no
-- 
1.7.8.3

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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