Re: [patch 1/2] mm: speculative get_page

[Hugh Dickins <hugh@veritas.com>]

A basic question I need to understand before going further...

On Wed, 26 Jul 2006, Nick Piggin wrote:
> + *
> + * This forms the core of the lockless pagecache locking protocol, where
> + * the lookup-side (eg. find_get_page) has the following pattern:
> + * 1. find page in radix tree
> + * 2. conditionally increment refcount
> + * 3. wait for PageNoNewRefs

(Better say
         wait while PageNoNewRefs
)

> + * 4. check the page is still in pagecache
> + *
> + * Remove-side (that cares about _count, eg. reclaim) has the following:
> + * A. SetPageNoNewRefs
> + * B. check refcount is correct
> + * C. remove page
> + * D. ClearPageNoNewRefs

Yes, I understand why remove_mapping and migrate_page_move_mapping
(on page) do the PageNoNewRefs business; but why do add_to_page_cache,
__add_to_swap_cache and migrate_page_move_mapping (on newpage) do it?

Hugh

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