linux-kernel - Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non

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Message-ID: <5190AE4F.4000103@cn.fujitsu.com>
Date:	Mon, 13 May 2013 17:11:43 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Minchan Kim <minchan@...nel.org>,
	Lin Feng <linfeng@...fujitsu.com>, akpm@...ux-foundation.org,
	bcrl@...ck.org, viro@...iv.linux.org.uk, khlebnikov@...nvz.org,
	walken@...gle.com, kamezawa.hiroyu@...fujitsu.com, riel@...hat.com,
	rientjes@...gle.com, isimatu.yasuaki@...fujitsu.com,
	wency@...fujitsu.com, laijs@...fujitsu.com, jiang.liu@...wei.com,
	zab@...hat.com, jmoyer@...hat.com, linux-mm@...ck.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of
 get_user_pages() called get_user_pages_non_movable()

Hi Mel,

On 02/06/2013 05:56 PM, Mel Gorman wrote:
>
> There is the possibility that callbacks could be introduced for
> migrate_unpin() and migrate_pin() that takes a list of PFN pairs
> (old,new). The unpin callback should release the old PFNs and barrier
> against any operations until the migrate_pfn() callback is called with
> the updated pfns to be repinned. Again it would fully depend on subsystems
> implementing it properly.
>
> The callback interface would be more robust but puts a lot more work on
> the driver side where your milage will vary.
>

I'm very interested in the "callback" way you said.

For memory hot-remove case, the aio pages are pined in memory and making
the pages cannot be offlined, furthermore, the pages cannot be removed.

IIUC, you mean implement migrate_unpin() and migrate_pin() callbacks in aio
subsystem, and call them when hot-remove code tries to offline pages, 
right ?

If so, I'm wondering where should we put this callback pointers ?
In struct page ?


It has been a long time since this topic was discussed. But to solve this
problem cleanly for hotplug guys and CMA guys, please give some more 
comments.

Thanks. :)

>
> To guarantee CMA can migrate pages pinned by drivers I think you need
> migrate-related callsbacks to unpin, barrier the driver until migration
> completes and repin.
>
> I do not know, or at least have no heard, of anyone working on such a
> scheme.
>
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