Date: Thu, 29 Nov 2012 23:55:02 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Lin Feng <linfeng@cn.fujitsu.com>
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
Subject: Re: [BUG REPORT] [mm-hotplug, aio] aio ring_pages can't be offlined
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On Fri, 30 Nov 2012 15:01:26 +0800 Lin Feng <linfeng@cn.fujitsu.com> wrote:

> 
> 
> On 11/30/2012 01:57 PM, Andrew Morton wrote:
> > 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?
> hi Andrew,
> 
> Yes, better to find an approach to solve them all.
>  
> But I'm worried about that if we just confine get_user_pages() to use 
> none-movable pages, it will drain the none-movable pages soon. Because
> there are many places using get_user_pages() such as some drivers. 

Obviously we shouldn't change get_user_pages() for all callers.

> IMHO in most cases get_user_pages() callers should release the pages soon, 
> so pages allocated from movable zone should be OK. But I'm not sure if
> we get such rule upon get_user_pages(). 
> And in other cases we specify get_user_pages() to allocate pages from
> none-movable zone. 
> 
> So could we add a zone-alloc flags when we call get_user_pages()?

Well, that's a fairly low-level implementation detail.  A more typical
approach would be to add a new get_user_pages_non_movable() or such. 
That would probably have the same signature as get_user_pages(), with
one additional argument.  Then get_user_pages() becomes a one-line
wrapper which passes in a particular value of that argument.

But that means we'd also have to add get_user_pages_fast_non_movable()
and things might become a bit stupid.  A better approach might be to
add a new library function which callers can use before (or after?)
calling get_user_pages[_fast]().

Unsure.  It's the sort of thing where one has to dive in and try a few
things.
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