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: <5192EE40.7060407@cn.fujitsu.com>
Date:	Wed, 15 May 2013 10:09:04 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Benjamin LaHaise <bcrl@...ck.org>, Mel Gorman <mgorman@...e.de>
CC:	Jeff Moyer <jmoyer@...hat.com>, 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()

Hi Benjamin, Mel,

Please see below.

On 05/14/2013 09:58 PM, Benjamin LaHaise wrote:
> 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.

In aio_complete(),

aio_complete() {
	......
	spin_lock_irqsave(&ctx->completion_lock, flags);
	//write the completion event.
	spin_unlock_irqrestore(&ctx->completion_lock, flags);
	......
}

So for this problem, I think we can hold ctx->completion_lock in the aio
callbacks to prevent aio subsystem accessing pages who are being migrated.

>
>> 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?
>

But as I said, for anonymous pages such as aio ring buffer, they don't have
address_space_operations. So where should we put the callbacks' pointers ?

Add something like address_space_operations to struct anon_vma ?

Thanks. :)






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