From d751e53a9cd77b0fa239fc843f838655c47484d8 Mon Sep 17 00:00:00 2001
From: Mel Gorman <mel@csn.ul.ie>
Date: Mon, 24 May 2010 14:32:20 -0700
Subject: [PATCH] --- yaml --- r: 198240 b: refs/heads/master c:
 3fe2011ff51e92500010a495df4be86745fbbda9 h: refs/heads/master v: v3

---
 [refs]             |  2 +-
 trunk/mm/migrate.c | 53 +++++++++++++++++++++++++++++++---------------
 2 files changed, 37 insertions(+), 18 deletions(-)

diff --git a/[refs] b/[refs]
index b715a7820701..48600204c979 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 67b9509b2c68ae38cecb83a239881cb0ddf087dc
+refs/heads/master: 3fe2011ff51e92500010a495df4be86745fbbda9
diff --git a/trunk/mm/migrate.c b/trunk/mm/migrate.c
index b114635962dc..4afd6fe3c074 100644
--- a/trunk/mm/migrate.c
+++ b/trunk/mm/migrate.c
@@ -485,7 +485,8 @@ static int fallback_migrate_page(struct address_space *mapping,
  *   < 0 - error code
  *  == 0 - success
  */
-static int move_to_new_page(struct page *newpage, struct page *page)
+static int move_to_new_page(struct page *newpage, struct page *page,
+						int remap_swapcache)
 {
 	struct address_space *mapping;
 	int rc;
@@ -520,10 +521,12 @@ static int move_to_new_page(struct page *newpage, struct page *page)
 	else
 		rc = fallback_migrate_page(mapping, newpage, page);
 
-	if (!rc)
-		remove_migration_ptes(page, newpage);
-	else
+	if (rc) {
 		newpage->mapping = NULL;
+	} else {
+		if (remap_swapcache)
+			remove_migration_ptes(page, newpage);
+	}
 
 	unlock_page(newpage);
 
@@ -540,6 +543,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	int rc = 0;
 	int *result = NULL;
 	struct page *newpage = get_new_page(page, private, &result);
+	int remap_swapcache = 1;
 	int rcu_locked = 0;
 	int charge = 0;
 	struct mem_cgroup *mem = NULL;
@@ -601,18 +605,33 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 		rcu_read_lock();
 		rcu_locked = 1;
 
-		/*
-		 * If the page has no mappings any more, just bail. An
-		 * unmapped anon page is likely to be freed soon but worse,
-		 * it's possible its anon_vma disappeared between when
-		 * the page was isolated and when we reached here while
-		 * the RCU lock was not held
-		 */
-		if (!page_mapped(page))
-			goto rcu_unlock;
+		/* Determine how to safely use anon_vma */
+		if (!page_mapped(page)) {
+			if (!PageSwapCache(page))
+				goto rcu_unlock;
 
-		anon_vma = page_anon_vma(page);
-		atomic_inc(&anon_vma->external_refcount);
+			/*
+			 * We cannot be sure that the anon_vma of an unmapped
+			 * swapcache page is safe to use because we don't
+			 * know in advance if the VMA that this page belonged
+			 * to still exists. If the VMA and others sharing the
+			 * data have been freed, then the anon_vma could
+			 * already be invalid.
+			 *
+			 * To avoid this possibility, swapcache pages get
+			 * migrated but are not remapped when migration
+			 * completes
+			 */
+			remap_swapcache = 0;
+		} else {
+			/*
+			 * Take a reference count on the anon_vma if the
+			 * page is mapped so that it is guaranteed to
+			 * exist when the page is remapped later
+			 */
+			anon_vma = page_anon_vma(page);
+			atomic_inc(&anon_vma->external_refcount);
+		}
 	}
 
 	/*
@@ -647,9 +666,9 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 
 skip_unmap:
 	if (!page_mapped(page))
-		rc = move_to_new_page(newpage, page);
+		rc = move_to_new_page(newpage, page, remap_swapcache);
 
-	if (rc)
+	if (rc && remap_swapcache)
 		remove_migration_ptes(page, page);
 rcu_unlock: