[PATCH 5/7] hugetlb: pin oldpage in page migration

[Naoya Horiguchi]

This patch introduces pinning the old page during page migration
to avoid freeing it before we complete copying.
This race condition can happen for privately mapped or anonymous hugepage.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Jun'ichi Nomura <j-nomura@xxxxxxxxxxxxx>
---
 mm/migrate.c |   26 +++++++++++++++++++++++---
 1 files changed, 23 insertions(+), 3 deletions(-)

diff --git v2.6.35-rc3-hwpoison/mm/migrate.c v2.6.35-rc3-hwpoison/mm/migrate.c
index 4205b1d..e4a381c 100644
--- v2.6.35-rc3-hwpoison/mm/migrate.c
+++ v2.6.35-rc3-hwpoison/mm/migrate.c
@@ -214,7 +214,7 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 
 	if (!mapping) {
 		/* Anonymous page without mapping */
-		if (page_count(page) != 1)
+		if (page_count(page) != 2 - PageHuge(page))
 			return -EAGAIN;
 		return 0;
 	}
@@ -224,7 +224,11 @@ static int migrate_page_move_mapping(struct address_space *mapping,
 	pslot = radix_tree_lookup_slot(&mapping->page_tree,
  					page_index(page));
 
-	expected_count = 2 + page_has_private(page);
+	/*
+	 * Hugepages are expected to have only one remained reference
+	 * from pagecache, because hugepages are not linked to LRU list.
+	 */
+	expected_count = 3 + page_has_private(page) - PageHuge(page);
 	if (page_count(page) != expected_count ||
 			(struct page *)radix_tree_deref_slot(pslot) != page) {
 		spin_unlock_irq(&mapping->tree_lock);
@@ -561,7 +565,11 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	if (!newpage)
 		return -ENOMEM;
 
-	if (page_count(page) == 1) {
+	/*
+	 * For anonymous hugepages, reference count is equal to mapcount,
+	 * so don't consider migration is done for anonymou hugepage.
+	 */
+	if (page_count(page) == 1 && !(PageHuge(page) && PageAnon(page))) {
 		/* page was freed from under us. So we are done. */
 		goto move_newpage;
 	}
@@ -644,6 +652,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	}
 
 	/*
+	 * It's reasonable to pin the old page until unmapping and copying
+	 * complete, because when the original page is an anonymous hugepage,
+	 * it will be freed in try_to_unmap() due to the fact that
+	 * all references of anonymous hugepage come from mapcount.
+	 * Although in the other cases no problem comes out without pinning,
+	 * it looks logically correct to do it.
+	 */
+	get_page(page);
+
+	/*
 	 * Corner case handling:
 	 * 1. When a new swap-cache page is read into, it is added to the LRU
 	 * and treated as swapcache but it has no rmap yet.
@@ -697,6 +715,8 @@ uncharge:
 unlock:
 	unlock_page(page);
 
+	put_page(page);
+
 	if (rc != -EAGAIN) {
  		/*
  		 * A page that has been migrated has all references
-- 
1.7.1

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