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

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Date:	Tue, 24 Jan 2012 17:44:49 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
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, 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.

> page_remove_rmap) so we can rely on it and push the correct charge type
> down to __mem_cgroup_uncharge_common from end_migration. The force flag
> was misleading anyway.

Kinda.  It had the effect of skipping the needless page_mapped() /
PageCgroupMigration() check, 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.
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