linux-kernel - Re: [BUGFIX][PATCH v2] add mem_cgroup_replace_page

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Date:	Fri, 9 Dec 2011 12:37:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cgroups@...r.kernel.org,
	"hannes@...xchg.org" <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [BUGFIX][PATCH v2] add mem_cgroup_replace_page_cache.

On Thu, 8 Dec 2011 16:18:29 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> commit ef6a3c6311 adds a function replace_page_cache_page(). This
> function replaces a page in radix-tree with a new page.
> At doing this, memory cgroup need to fix up the accounting information.
> memcg need to check PCG_USED bit etc.
> 
> In some(many?) case, 'newpage' is on LRU before calling replace_page_cache().
> So, memcg's LRU accounting information should be fixed, too.
> 
> This patch adds mem_cgroup_replace_page_cache() and removing old hooks.
> In that function, old pages will be unaccounted without touching res_counter
> and new page will be accounted to the memcg (of old page). At overwriting
> pc->mem_cgroup of newpage, take zone->lru_lock and avoid race with
> LRU handling.
> 
> Background:
>   replace_page_cache_page() is called by FUSE code in its splice() handling.
>   Here, 'newpage' is replacing oldpage but this newpage is not a newly allocated
>   page and may be on LRU. LRU mis-accounting will be critical for memory cgroup
>   because rmdir() checks the whole LRU is empty and there is no account leak.
>   If a page is on the other LRU than it should be, rmdir() will fail.
> 
> Changelog: v1 -> v2
>   - fixed mem_cgroup_disabled() check missing.
>   - added comments.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  include/linux/memcontrol.h |    6 ++++++
>  mm/filemap.c               |   18 ++----------------
>  mm/memcontrol.c            |   44 ++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 52 insertions(+), 16 deletions(-)

It's a relatively intrusive patch and I'm a bit concerned about
feeding it into 3.2.

How serious is the bug, and which kernel version(s) do you think we
should fix it in?

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