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

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Message-Id: <20101217090103.2a9ca19a.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 17 Dec 2010 09:01:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add replace_page_cache_page() function

On Thu, 16 Dec 2010 13:05:44 +0100
Miklos Szeredi <miklos@...redi.hu> wrote:

> On Thu, 16 Dec 2010, KAMEZAWA Hiroyuki wrote:

> > Hmm, then, the page will be recharged to "current" instead of the memcg
> > where "old" was under control. Is this design ? If so, why ?
> 
> No, I just haven't thought about it.
> 
> Porbably charging "new" to where "old" was charged is the logical
> thing to do here.
> 
> > 
> > In mm/migrate.c, following is called.
> > 
> > 	 charge = mem_cgroup_prepare_migration(page, newpage, &mem);
> > 	....do migration....
> >         if (!charge)
> >                 mem_cgroup_end_migration(mem, page, newpage);
> > 
> > BTW, off topic, in fuse/dev.c
> > 
> > add_to_page_cache_locked(page)
> 
> This is the call which the above patch replaces with
> replace_page_cache_page().  So if I fix replace_page_cache_page() to
> charge "newpage" to the correct memory cgroup, that should solve all
> problems, no?
> 
No. memory cgroup expects all pages should be found on LRU. But, IIUC,
pages on this radix-tree will not be on LRU. So, memory cgroup can't find
it at destroying cgroup and can't reduce "usage" of resource to be 0.
This makes rmdir() returns -EBUSY.

I'm sorry if this page will be on LRU, somewhere.

Thanks,
-Kame

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