linux-kernel - Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages

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Date:	Thu, 22 Apr 2010 09:11:04 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache 
	pages

On Thu, Apr 22, 2010 at 8:59 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 21 Apr 2010 09:30:20 -0500 (CDT)
> Christoph Lameter <cl@...ux-foundation.org> wrote:
>
>> On Tue, 20 Apr 2010, Mel Gorman wrote:
>>
>> > @@ -520,10 +521,12 @@ static int move_to_new_page(struct page *newpage, struct page *page)
>> >     else
>> >             rc = fallback_migrate_page(mapping, newpage, page);
>> >
>> > -   if (!rc)
>> > -           remove_migration_ptes(page, newpage);
>> > -   else
>> > +   if (rc) {
>> >             newpage->mapping = NULL;
>> > +   } else {
>> > +           if (remap_swapcache)
>> > +                   remove_migration_ptes(page, newpage);
>> > +   }
>>
>> You are going to keep the migration ptes after the page has been unlocked?
>> Or is remap_swapcache true if its not a swapcache page?
>>
>> Maybe you meant
>>
>> if (!remap_swapcache)
>>
>
> Ah....Can I confirm my understanding ?
>
> remap_swapcache == true only when
>  The old page was ANON && it is not mapped. && it is SwapCache.
>
> We do above check under lock_page(). So, this SwapCache is never mapped until
> we release lock_page() on the old page. So, we don't use migration_pte in
> this case because try_to_unmap() do nothing and don't need to call
> remove_migration_pte().

Yes. so I thought what kinds of race happened.
Firstly I doubt fork and migration. but It isn't.
I can't understand how this bug happens.
Apparently, We have been missed something.
I will look into this further.


-- 
Kind regards,
Minchan Kim
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