[PATCH 4/4] correct locking functions of hugepage migration routine

[Naoya Horiguchi]

For the migration of PAGE_SIZE pages, we can choose to continue to do
migration with "force" switch even if the old page has page lock held.
But for hugepage, I/O of subpages are not necessarily completed
in ascending order, which can cause race.
So we make migration fail then for safety.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
---
 mm/migrate.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 7f9a37c..43347e1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -820,11 +820,14 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 
 	rc = -EAGAIN;
 
-	if (!trylock_page(hpage)) {
-		if (!force)
-			goto out;
-		lock_page(hpage);
-	}
+	/*
+	 * If some subpages are locked, it can cause race condition.
+	 * So then we return from migration and try again.
+	 */
+	if (!trylock_huge_page(hpage))
+		goto out;
+
+	wait_on_huge_page_writeback(hpage);
 
 	if (PageAnon(hpage)) {
 		rcu_read_lock();
@@ -855,7 +858,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	if (rcu_locked)
 		rcu_read_unlock();
 out:
-	unlock_page(hpage);
+	unlock_huge_page(hpage);
 
 	if (rc != -EAGAIN)
 		put_page(hpage);
-- 
1.7.2.1

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