Re: [RFC 5/6] Swapless V1: Rip out swap migration code

[Christoph Lameter <clameter@sgi.com>]

On Wed, 5 Apr 2006, KAMEZAWA Hiroyuki wrote:

> On Tue, 4 Apr 2006 19:45:49 -0700 (PDT)
> Christoph Lameter <clameter@sgi.com> wrote:
> 
> > > My concern is refcnt handling of SWP_TYPE_MIGRATION pages, but maybe no problem.
> > 
> > What are the exact concerns?
> > 
> When a page is converted into SWP_TYPE_MIGRATION, changed pte entry
> implicitly points old page. This introduces the state 'a page is referred 
> but no refcnt'. if mmap_sem is held, this is maybe no problem. 
> but looks a bit dangerous.

We have increased the refcnt on the page (see isolate_lru_page()) and the 
page is locked when  SWP_TYPE_MIGRATION is used. So there is a refcnt.

> > > I think adding SWP_TYPE_MIGRATION consideration to free_swap_and_cache() is
> > > enough against anon_vma vanishing. Because remove_migration_ptes() compares 
> > > old pte entry with old page's pfn, a page cannot be remapped into old place
> > > when anon_vma has gone. This is my first impression.
> > 
> > However, the last process containing the page may terminate and free the 
> > page, while we migrate. The SWAP_TYPE_MIGRATION pte will be rewoved 
> > together with the anonvma if no lock is held on mmap_sem. 
> yes. 
> 
> > Then remove_migration_ptes() cannot obtain a anon_vma. So it would break 
> > without holding mmap_sem. We could fix this if we could somehow know that 
> > the last process mapping the page vanished and skip 
> > remove_migration_ptes().
> > 
> 
> Hmm, I'm not sure but how about this way ?
> 1. don't drop refcnt in try_to_unmap_one() when changing a page to 
>    SWP_TYPE_MIGRATION. because it is referred. (rmap should be removed ?)

Then we would have a page with mapcounts but there are no real ptes 
pointing to the page. It would be a strange condition for the page. 

Moreover, a process may fork or terminate while we migrate. Forking may 
increase the refcnt and termination may decrease it. We do not keep
refcnts for the SWP_TYPE_MIGRATION entry but rely on the reverse maps. So 
we may end up with a messed up mapcount if we do not drop the refcnts.

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