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SubjectRe: [PATCH 04/14] mm,migration: Allow the migration of PageSwapCache pages
On Wed, 21 Apr 2010, Mel Gorman wrote:

> > > 2. Is the BUG_ON check in
> > > include/linux/swapops.h#migration_entry_to_page() now wrong? (I
> > > think yes, but I'm not sure and I'm having trouble verifying it)
> >
> > The bug check ensures that migration entries only occur when the page
> > is locked. This patch changes that behavior. This is going too oops
> > therefore in unmap_and_move() when you try to remove the migration_ptes
> > from an unlocked page.
> >
> It's not unmap_and_move() that the problem is occurring on but during a
> page fault - presumably in do_swap_page but I'm not 100% certain.

remove_migration_pte() calls migration_entry_to_page(). So it must do that
only if the page is still locked.

You need to ensure that the page is not unlocked in move_to_new_page() if
the migration ptes are kept.

move_to_new_page() only unlocks the new page not the original page. So that is safe.

And it seems that the old page is also unlocked in unmap_and_move() only
after the migration_ptes have been removed? So we are fine after all...?

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