Re: [RFC] 2/4 Migration Cache - add mm checks

[Christoph Lameter <clameter@engr.sgi.com>]

On Fri, 17 Feb 2006, Lee Schermerhorn wrote:

> Index: linux-2.6.16-rc3-mm1/mm/vmscan.c
> ===================================================================
> --- linux-2.6.16-rc3-mm1.orig/mm/vmscan.c	2006-02-15 10:50:43.000000000 -0500
> +++ linux-2.6.16-rc3-mm1/mm/vmscan.c	2006-02-15 10:50:53.000000000 -0500
> @@ -457,11 +457,19 @@ static unsigned long shrink_page_list(st
>  		 * Anonymous process memory has backing store?
>  		 * Try to allocate it some swap space here.
>  		 */
> -		if (PageAnon(page) && !PageSwapCache(page)) {
> -			if (!sc->may_swap)
> +		if (PageAnon(page)) {
> +			if (!PageSwapCache(page)) {
> +				if (!sc->may_swap)
> +					goto keep_locked;
> +				if (!add_to_swap(page, GFP_ATOMIC))
> +					goto activate_locked;
> +			} else if (page_is_migration(page)) {
> +				/*
> +				 * For now, skip migration cache pages.
> +				 * TODO:  move to swap cache [difficult?]
> +				 */
>  				goto keep_locked;
> -			if (!add_to_swap(page, GFP_ATOMIC))
> -				goto activate_locked;
> +			}
>  		}
>  #endif /* CONFIG_SWAP */


Would it not be simpler to modify add_to_swap to switch from migration
pte to a real swap pte or simply fail? Then you wont have to touch 
shrink_page().

> Index: linux-2.6.16-rc3-mm1/mm/rmap.c
> ===================================================================
> --- linux-2.6.16-rc3-mm1.orig/mm/rmap.c	2006-02-15 10:50:43.000000000 -0500
> +++ linux-2.6.16-rc3-mm1/mm/rmap.c	2006-02-15 10:50:53.000000000 -0500
> @@ -232,7 +232,13 @@ void remove_from_swap(struct page *page)
>  
>  	spin_unlock(&anon_vma->lock);
>  
> -	delete_from_swap_cache(page);
> +	if (PageSwapCache(page))
> +		delete_from_swap_cache(page);
> +	/*
> +	 * if page was in migration cache, it will have been
> +	 * removed when the last swap pte referencing the entry
> +	 * was removed by the loop above.
> +	 */
>  }
>  EXPORT_SYMBOL(remove_from_swap);
>  #endif

Hmmm. That points to inconsistent behavior of the swap functions in case 
these are working on the migration cache. Could you keep PageSwapCache
until delete_from_swap_cache is called?

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