linux-kernel - Re: [PATCH 1/3] When migrate_pages returns 0, all pages must have been released

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Date:	Thu, 20 Jan 2011 12:49:15 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH 1/3] When migrate_pages returns 0, all pages must have
 been released

On Thu, 20 Jan 2011, Andrea Arcangeli wrote:

> Hello,
>
> On Thu, Jan 20, 2011 at 11:30:35AM -0600, Christoph Lameter wrote:
> > On Fri, 21 Jan 2011, Minchan Kim wrote:
> >
> > > diff --git a/mm/migrate.c b/mm/migrate.c
> > > index 46fe8cc..7d34237 100644
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -772,6 +772,7 @@ uncharge:
> > >  unlock:
> > >  	unlock_page(page);
> > >
> > > +move_newpage:
> > >  	if (rc != -EAGAIN) {
> > >   		/*
> > >   		 * A page that has been migrated has all references
> > > @@ -785,8 +786,6 @@ unlock:
> > >  		putback_lru_page(page);
> > >  	}
> > >
> > > -move_newpage:
> > > -
> > >  	/*
> > >  	 * Move the new page to the LRU. If migration was not successful
> > >  	 * then this will free the page.
> > >
> >
> > What does this do? Not covered by the description.
>
> It makes a difference for the two goto move_newpage, when rc =
> 0. Otherwise the function will return 0, despite
> putback_lru_page(page) wasn't called (and the caller of migrate_pages
> won't call putback_lru_pages if migrate_pages returned 0).

Think about the difference:

Moving the move_newpage will now cause another removal and freeing of the
page if rc != -EAGAIN.

The first goto move_newpage (because page count is 1) will now mean that
the page is freed twice. One because of the rc != EAGAIN branch and then
another time by the following putback_lru_page().


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