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: <20130514135850.GG13845@kvack.org>
Date:	Tue, 14 May 2013 09:58:50 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Tang Chen <tangchen@...fujitsu.com>
Cc:	Jeff Moyer <jmoyer@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Minchan Kim <minchan@...nel.org>,
	Lin Feng <linfeng@...fujitsu.com>, akpm@...ux-foundation.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, 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()

On Tue, May 14, 2013 at 09:24:58AM +0800, Tang Chen wrote:
> Hi Mel, Benjamin, Jeff,
> 
> On 05/13/2013 11:01 PM, Benjamin LaHaise wrote:
> >On Mon, May 13, 2013 at 10:54:03AM -0400, Jeff Moyer wrote:
> >>How do you propose to move the ring pages?
> >
> >It's the same problem as doing a TLB shootdown: flush the old pages from
> >userspace's mapping, copy any existing data to the new pages, then
> >repopulate the page tables.  It will likely require the addition of
> >address_space_operations for the mapping, but that's not too hard to do.
> >
> 
> I think we add migrate_unpin() callback to decrease page->count if 
> necessary,
> and migrate the page to a new page, and add migrate_pin() callback to pin
> the new page again.

You can't just decrease the page count for this to work.  The pages are 
pinned because aio_complete() can occur at any time and needs to have a 
place to write the completion events.  When changing pages, aio has to 
take the appropriate lock when changing one page for another.

> The migrate procedure will work just as before. We use callbacks to 
> decrease
> the page->count before migration starts, and increase it when the migration
> is done.
> 
> And migrate_pin() and migrate_unpin() callbacks will be added to
> struct address_space_operations.

I think the existing migratepage operation in address_space_operations can 
be used.  Does it get called when hot unplug occurs?  That is: is testing 
with the migrate_pages syscall similar enough to the memory removal case?

		-ben

> Is that right ?
> 
> If so, I'll be working on it.
> 
> Thanks. :)

-- 
"Thought is the essence of where you are now."
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