[PATCH] Swapless page migration: modify core logic

Use the migration entries for page migration

This modifies the migration code to use the new migration entries.  It now
becomes possible to migrate anonymous pages without having to add a swap
entry.

We add a couple of new functions to replace migration entries with the proper
ptes.

We cannot take the tree_lock for migrating anonymous pages anymore.  However,
we know that we hold the only remaining reference to the page when the page
count reaches 1.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/Kconfig

@@ -138,8 +138,8 @@ config SPLIT_PTLOCK_CPUS
 #
 config MIGRATION
 	bool "Page migration"
-	def_bool y if NUMA
+	def_bool y
-	depends on SWAP && NUMA
+	depends on NUMA
 	help
 	  Allows the migration of the physical location of pages of processes
 	  while the virtual addresses are not changed. This is useful for

mm/migrate.c

@@ -254,14 +254,20 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 {
 	struct page **radix_pointer;
 
+	if (!mapping) {
+		/* Anonymous page */
+		if (page_count(page) != 1)
+			return -EAGAIN;
+		return 0;
+	}
+
 	write_lock_irq(&mapping->tree_lock);
 
 	radix_pointer = (struct page **)radix_tree_lookup_slot(
 						&mapping->page_tree,
 						page_index(page));
 
-	if (!page_mapping(page) ||
+	if (page_count(page) != 2 + !!PagePrivate(page) ||
-			page_count(page) != 2 + !!PagePrivate(page) ||
 			*radix_pointer != page) {
 		write_unlock_irq(&mapping->tree_lock);
 		return -EAGAIN;
@@ -271,10 +277,12 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 	 * Now we know that no one else is looking at the page.
 	 */
 	get_page(newpage);
+#ifdef CONFIG_SWAP
 	if (PageSwapCache(page)) {
 		SetPageSwapCache(newpage);
 		set_page_private(newpage, page_private(page));
 	}
+#endif
 
 	*radix_pointer = newpage;
 	__put_page(page);
@@ -308,7 +316,9 @@ static void migrate_page_copy(struct page *newpage, struct page *page)
 		set_page_dirty(newpage);
  	}
 
+#ifdef CONFIG_SWAP
 	ClearPageSwapCache(page);
+#endif
 	ClearPageActive(page);
 	ClearPagePrivate(page);
 	set_page_private(page, 0);
@@ -353,16 +363,6 @@ int migrate_page(struct address_space *mapping,
 		return rc;
 
 	migrate_page_copy(newpage, page);
-
-	/*
-	 * Remove auxiliary swap entries and replace
-	 * them with real ptes.
-	 *
-	 * Note that a real pte entry will allow processes that are not
-	 * waiting on the page lock to use the new page via the page tables
-	 * before the new page is unlocked.
-	 */
-	remove_from_swap(newpage);
 	return 0;
 }
 EXPORT_SYMBOL(migrate_page);
@@ -530,23 +530,7 @@ redo:
 				goto unlock_page;
 
 		/*
-		 * Establish swap ptes for anonymous pages or destroy pte
+		 * Establish migration ptes or remove ptes
-		 * maps for files.
-		 *
-		 * In order to reestablish file backed mappings the fault handlers
-		 * will take the radix tree_lock which may then be used to stop
-	  	 * processses from accessing this page until the new page is ready.
-		 *
-		 * A process accessing via a swap pte (an anonymous page) will take a
-		 * page_lock on the old page which will block the process until the
-		 * migration attempt is complete. At that time the PageSwapCache bit
-		 * will be examined. If the page was migrated then the PageSwapCache
-		 * bit will be clear and the operation to retrieve the page will be
-		 * retried which will find the new page in the radix tree. Then a new
-		 * direct mapping may be generated based on the radix tree contents.
-		 *
-		 * If the page was not migrated then the PageSwapCache bit
-		 * is still set and the operation may continue.
 		 */
 		rc = -EPERM;
 		if (try_to_unmap(page, 1) == SWAP_FAIL)
@@ -569,9 +553,9 @@ redo:
 		 */
 		mapping = page_mapping(page);
 		if (!mapping)
-			goto unlock_both;
+			rc = migrate_page(mapping, newpage, page);
 
-		if (mapping->a_ops->migratepage)
+		else if (mapping->a_ops->migratepage)
 			/*
 			 * Most pages have a mapping and most filesystems
 			 * should provide a migration function. Anonymous
@@ -584,10 +568,15 @@ redo:
 		else
 			rc = fallback_migrate_page(mapping, newpage, page);
 
-unlock_both:
+		if (!rc)
+			remove_migration_ptes(page, newpage);
+
 		unlock_page(newpage);
 
 unlock_page:
+		if (rc)
+			remove_migration_ptes(page, page);
+
 		unlock_page(page);
 
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