linux-kernel - Re: [PATCH v4] mm: add replace_page_cache

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Message-Id: <20110119092733.4927f935.nishimura@mxp.nes.nec.co.jp>
Date:	Wed, 19 Jan 2011 09:27:33 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>, minchan.kim@...il.com,
	kamezawa.hiroyu@...fujitsu.com, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH v4] mm: add replace_page_cache_page() function

On Tue, 18 Jan 2011 15:28:44 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Tue, 18 Jan 2011 12:18:11 +0100
> Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > +int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
> > +{
> > +	int error;
> > +	struct mem_cgroup *memcg = NULL;
> 
> I'm suspecting that the unneeded initialisation was added to suppress a
> warning?
> 
No.
It's necessary for mem_cgroup_{prepare|end}_migration().
mem_cgroup_prepare_migration() will return without doing anything in
"if (mem_cgroup_disabled()" case(iow, "memcg" is not overwritten),
but mem_cgroup_end_migration() depends on the value of "memcg" to decide
whether prepare_migration has succeeded or not.
This may not be a good implementation, but IMHO I'd like to to initialize
valuable before using it in general.

Thanks,
Daisuke Nishimura.
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