[PATCH 9/13] Do not group pages by mobility type on low memory systems

[Mel Gorman <mel@csn.ul.ie>]

Where there are fewer than one pageblock in the system per mobility
type mixing is inevitable and any attempt to prevent it will fail
in a costly manner. This patch checks the size of vm_total_pages in
build_all_zonelists(). If there are not enough areas,  mobility is effectivly
disabled by considering all allocations as the same type (UNMOVABLE).
This is achived via a __read_mostly flag.

This patch removes any need to disable grouping pages by mobility at
compile time.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |   25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff -rup -X /usr/src/patchset-0.6/bin//dontdiff linux-2.6.23-rc5-008-move-free-pages-between-lists-on-steal/mm/page_alloc.c linux-2.6.23-rc5-009-do-not-group-pages-by-mobility-type-on-low-memory-systems/mm/page_alloc.c
--- linux-2.6.23-rc5-008-move-free-pages-between-lists-on-steal/mm/page_alloc.c	2007-09-02 16:21:09.000000000 +0100
+++ linux-2.6.23-rc5-009-do-not-group-pages-by-mobility-type-on-low-memory-systems/mm/page_alloc.c	2007-09-02 16:21:30.000000000 +0100
@@ -154,8 +154,13 @@ int nr_node_ids __read_mostly = MAX_NUMN
 EXPORT_SYMBOL(nr_node_ids);
 #endif
 
+int page_group_by_mobility_disabled __read_mostly;
+
 static inline int get_pageblock_migratetype(struct page *page)
 {
+	if (unlikely(page_group_by_mobility_disabled))
+		return MIGRATE_UNMOVABLE;
+
 	return get_pageblock_flags_group(page, PB_migrate, PB_migrate_end);
 }
 
@@ -169,6 +174,10 @@ static inline int allocflags_to_migratet
 {
 	WARN_ON((gfp_flags & GFP_MOVABLE_MASK) == GFP_MOVABLE_MASK);
 
+	if (unlikely(page_group_by_mobility_disabled))
+		return MIGRATE_UNMOVABLE;
+
+	/* Cluster based on mobility */
 	return (((gfp_flags & __GFP_MOVABLE) != 0) << 1) |
 		((gfp_flags & __GFP_RECLAIMABLE) != 0);
 }
@@ -2294,9 +2303,23 @@ void build_all_zonelists(void)
 		/* cpuset refresh routine should be here */
 	}
 	vm_total_pages = nr_free_pagecache_pages();
-	printk("Built %i zonelists in %s order.  Total pages: %ld\n",
+	/*
+	 * Disable grouping by mobility if the number of pages in the
+	 * system is too low to allow the mechanism to work. It would be
+	 * more accurate, but expensive to check per-zone. This check is
+	 * made on memory-hotadd so a system can start with mobility
+	 * disabled and enable it later
+	 */
+	if (vm_total_pages < (pageblock_nr_pages * MIGRATE_TYPES))
+		page_group_by_mobility_disabled = 1;
+	else
+		page_group_by_mobility_disabled = 0;
+
+	printk("Built %i zonelists in %s order, mobility grouping %s.  "
+		"Total pages: %ld\n",
 			num_online_nodes(),
 			zonelist_order_name[current_zonelist_order],
+			page_group_by_mobility_disabled ? "off" : "on",
 			vm_total_pages);
 #ifdef CONFIG_NUMA
 	printk("Policy zone: %s\n", zone_names[policy_zone]);

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