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

[Christoph Lameter <clameter@sgi.com>]

On Wed, 5 Apr 2006, KAMEZAWA Hiroyuki wrote:

> 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. 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().

> My concern is refcnt handling of SWP_TYPE_MIGRATION pages, but maybe no problem.

What are the exact concerns?


> Note: unuse_vma() doesn't check what pte entry contains.

unuse_vma() relies on the mapping via swap space that will no longer exist 
with the new code.

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