[RFC] 4/4 Migration Cache - use for direct migration

[RFC] 4/4 Migration Cache - use for direct migration

[Lee Schermerhorn]

Migration Cache "V8" 4/4

This patch hooks the migration cache up to direct page migration.
If a destination page exists, and the old page is not already in
a swap cache, we place it in the migration cache instead.

If direct migration is successful, the page will be removed by
remove_from_swap(), et al.  If migration is not successful after
4 or 5 passes, migrate_pages() drops back to swapping.  In this
case, we must move the page from the migration cache to the swap
cache.

Signed-off-by:  Lee Schermerhorn <lee.schermerhorn@hp.com>

Index: linux-2.6.16-rc3-mm1/mm/vmscan.c
===================================================================
--- linux-2.6.16-rc3-mm1.orig/mm/vmscan.c	2006-02-15 10:50:59.000000000 -0500
+++ linux-2.6.16-rc3-mm1/mm/vmscan.c	2006-02-15 10:51:09.000000000 -0500
@@ -911,7 +911,12 @@ redo:
 		 * preserved.
 		 */
 		if (PageAnon(page) && !PageSwapCache(page)) {
-			if (!add_to_swap(page, GFP_KERNEL)) {
+			if (!to) {
+				if (!add_to_swap(page, GFP_KERNEL)) {
+					rc = -ENOMEM;
+					goto unlock_page;
+				}
+			} else if (add_to_migration_cache(page, GFP_KERNEL)) {
 				rc = -ENOMEM;
 				goto unlock_page;
 			}
@@ -989,6 +994,12 @@ redo:
 			 */
 			unlock_page(newpage);
 			newpage = NULL;
+			if (page_is_migration(page)) {
+				if (!migration_move_to_swap(page)) {
+					rc = -ENOMEM;
+					goto unlock_page;
+				}
+			}
 			rc = swap_page(page);
 			goto next;
 		}


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Re: [RFC] 4/4 Migration Cache - use for direct migration

[Christoph Lameter]

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 10:50:59.000000000 -0500
> +++ linux-2.6.16-rc3-mm1/mm/vmscan.c	2006-02-15 10:51:09.000000000 -0500
> @@ -911,7 +911,12 @@ redo:
>  		 * preserved.
>  		 */
>  		if (PageAnon(page) && !PageSwapCache(page)) {
> -			if (!add_to_swap(page, GFP_KERNEL)) {
> +			if (!to) {
> +				if (!add_to_swap(page, GFP_KERNEL)) {
> +					rc = -ENOMEM;
> +					goto unlock_page;
> +				}
> +			} else if (add_to_migration_cache(page, GFP_KERNEL)) {
>  				rc = -ENOMEM;
>  				goto unlock_page;
>  			}

Hmmm.... maybe add another parameter to add_to_swap instead? This seems to 
be duplicating some code.

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Re: [RFC] 4/4 Migration Cache - use for direct migration

[Lee Schermerhorn]

On Fri, 2006-02-17 at 08:35 -0800, Christoph Lameter wrote:
> 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 10:50:59.000000000 -0500
> > +++ linux-2.6.16-rc3-mm1/mm/vmscan.c	2006-02-15 10:51:09.000000000 -0500
> > @@ -911,7 +911,12 @@ redo:
> >  		 * preserved.
> >  		 */
> >  		if (PageAnon(page) && !PageSwapCache(page)) {
> > -			if (!add_to_swap(page, GFP_KERNEL)) {
> > +			if (!to) {
> > +				if (!add_to_swap(page, GFP_KERNEL)) {
> > +					rc = -ENOMEM;
> > +					goto unlock_page;
> > +				}
> > +			} else if (add_to_migration_cache(page, GFP_KERNEL)) {
> >  				rc = -ENOMEM;
> >  				goto unlock_page;
> >  			}
> 
> Hmmm.... maybe add another parameter to add_to_swap instead? This seems to 
> be duplicating some code.
> 

Could do.  The value of which would depend on 'to' in this context.  

This would require a change to shrink_list() in vmscan which you wanted
to avoid in comment on previous mail, or we could leave add_to_swap() as
a wrapper over the current one renamed to __add_to_swap or such with the
extra parameter.

Could also change the code above to use a single if:

if ((!to && !add_to_swap()) || (to && add_to_migration_cache())) ...

I'm not too clear on what passes for "readability" and how that weighs
against this level code duplication. ;-)

But, in general, I agree with the notion of hiding the details or
even existence of the migration cache behind the swap interfaces.


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