linux-kernel - Re: [BUG REPORT] [mm-hotplug, aio] aio ring

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Date:	Thu, 29 Nov 2012 21:57:49 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Lin Feng <linfeng@...fujitsu.com>
Cc:	viro@...iv.linux.org.uk, bcrl@...ck.org,
	kamezawa.hiroyu@...fujitsu.com, mhocko@...e.cz, hughd@...gle.com,
	cl@...ux.com, mgorman@...e.de, minchan@...nel.org,
	isimatu.yasuaki@...fujitsu.com, laijs@...fujitsu.com,
	wency@...fujitsu.com, tangchen@...fujitsu.com,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined

On Fri, 30 Nov 2012 11:42:05 +0800 Lin Feng <linfeng@...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?
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