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SubjectRe: [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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