From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined
Date: Thu, 29 Nov 2012 21:57:49 -0800
Message-ID: <20121129215749.acfd872a.akpm@linux-foundation.org>
References: <1354172098-5691-1-git-send-email-linfeng@cn.fujitsu.com>
	<20121129153930.477e9709.akpm@linux-foundation.org>
	<50B82B0D.8010206@cn.fujitsu.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: viro@zeniv.linux.org.uk, bcrl@kvack.org,
	kamezawa.hiroyu@jp.fujitsu.com, mhocko@suse.cz, hughd@google.com,
	cl@linux.com, mgorman@suse.de, minchan@kernel.org,
	isimatu.yasuaki@jp.fujitsu.com, laijs@cn.fujitsu.com,
	wency@cn.fujitsu.com, tangchen@cn.fujitsu.com,
	linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
To: Lin Feng <linfeng@cn.fujitsu.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <50B82B0D.8010206@cn.fujitsu.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@cn.fujitsu.com> wrote:

> hi Andrew,
> 
> On 11/30/2012 07:39 AM, Andrew Morton wrote:
> > Tricky.
> > 
> > I expect the same problem would occur with pages which are under
> > O_DIRECT I/O.  Obviously O_DIRECT pages won't be pinned for such long
> > periods, but the durations could still be lengthy (seconds).
> the offline retry timeout duration is 2 minutes, so to O_DIRECT pages 
> seem maybe not a problem for the moment.
> > 
> > Worse is a futex page, which could easily remain pinned indefinitely.
> > 
> > The best I can think of is to make changes in or around
> > get_user_pages(), to steal the pages from userspace and replace them
> > with non-movable ones before pinning them.  The performance cost of
> > something like this would surely be unacceptable for direct-io, but
> > maybe OK for the aio ring and futexes.
> thanks for your advice.
> I want to limit the impact as little as possible, as mentioned above,
> direct-io seems not a problem, we needn't touch them. Maybe we can 
> just change the use of get_user_pages()(in or around) such as aio 
> ring pages. I will try to find a way to do this.

What about futexes?