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Date: Fri, 14 Apr 2006 11:31:04 -0700
From: Andrew Morton <akpm@osdl.org>
To: Christoph Lameter <clameter@sgi.com>
Cc: hugh@veritas.com, linux-kernel@vger.kernel.org, lee.schermerhorn@hp.com,
       linux-mm@kvack.org, taka@valinux.co.jp, marcelo.tosatti@cyclades.com,
       kamezawa.hiroyu@jp.fujitsu.com
Subject: Re: Implement lookup_swap_cache for migration entries
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Christoph Lameter <clameter@sgi.com> wrote:
>
> This undoes the optimization that resulted in a yield in do_swap_cache().
> do_swap_cache() stays as is. Instead we convert the migration entry to
> a page * in lookup_swap_cache.
> 
> For the non swap case we need a special macro version of lookup_swap_cache
> that is only capable of handling migration cache entries.
> 
> ...
>
> @@ -305,6 +306,12 @@ struct page * lookup_swap_cache(swp_entr
>  {
>  	struct page *page;
>  
> +	if (is_migration_entry(entry)) {
> +		page = migration_entry_to_page(entry);
> +		get_page(page);
> +		return page;
> +	}

What locking ensures that the state of `entry' remains unaltered across the
is_migration_entry() and migration_entry_to_page() calls?

>  
> +/*
> + * Must use a macro for lookup_swap_cache since the functions
> + * used are only available in certain contexts.
> + */
> +#define lookup_swap_cache(__swp)				\
> +({	struct page *p = NULL;					\
> +	if (is_migration_entry(__swp)) {			\
> +		p = migration_entry_to_page(__swp);		\
> +		get_page(p);					\
> +	}							\
> +	p;							\
> +})

hm.  Can nommu do any of this?