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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Date:	Tue, 14 May 2013 11:55:31 +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 05/13/2013 05:19 PM, Mel Gorman wrote:
>> 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 ?
>>
>
> No, I would expect the callbacks to be part the address space operations
> which can be found via page->mapping.
>

Two more problems I don't quite understand.

1. For an anonymous page, it has no address_space, and no address space
    operation. But the aio ring problem just happened when dealing with
    anonymous pages. Please refer to:
    (https://lkml.org/lkml/2012/11/29/69)

    If we put the the callbacks in page->mapping->a_ops, the anonymous 
pages
    won't be able to use them.

    And we cannot give a default callback because the situation we are 
dealing
    with is a special situation.

    So where to put the callback for anonymous pages ?


2. How to find out the reason why page->count != 1 in 
migrate_page_move_mapping() ?

    In the problem we are dealing with, get_user_pages() is called to 
pin the pages
    in memory. And the pages are migratable. So we want to decrease the 
page->count.

    But get_user_pages() is not the only reason leading to page->count 
increased.
    How can I know when should decrease teh page->count or when should not ?

    The way I can figure out is to assign the callback pointer in 
get_user_pages()
    because it is get_user_pages() who pins the pages.


Thanks. :)








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