linux-kernel - Re: [PATCH 06/10] migrate: add hugepage migration code to move

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Date:	Tue, 26 Mar 2013 16:37:44 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/10] migrate: add hugepage migration code to
 move_pages()

On Tue, Mar 26, 2013 at 11:02:21AM +0100, Michal Hocko wrote:
> On Tue 26-03-13 03:06:18, Naoya Horiguchi wrote:
> > On Mon, Mar 25, 2013 at 02:36:44PM +0100, Michal Hocko wrote:
> > > On Fri 22-03-13 16:23:51, Naoya Horiguchi wrote:
> > > > @@ -1164,6 +1175,12 @@ static int do_move_page_to_node_array(struct mm_struct *mm,
> > > [...]
> > > >  				!migrate_all)
> > > >  			goto put_and_set;
> > > >  
> > > > +		if (PageHuge(page)) {
> > > > +			get_page(page);
> > > > +			list_move_tail(&page->lru, &pagelist);
> > > > +			goto put_and_set;
> > > > +		}
> > > 
> > > Why do you take an additional reference here? You have one from
> > > follow_page already.
> > 
> > For normal pages, follow_page(FOLL_GET) takes a refcount and
> > isolate_lru_page() takes another one, so I think the same should
> > be done for hugepages. Refcounting of this function looks tricky,
> > and I'm not sure why existing code does like that.
> 
> Ohh, I see. But the whole reference is taken just to release it in goto
> put_and_set because isolate_lru_page elevates reference count because
> other users require that. I think you do not have to mimic this behavior
> here and you can drop get_page and use goto set_status.

OK, thanks.
Naoya
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