[PATCH 1/9] mm: rcu read lock for getting reference on pages in migration_entry_wait()

[Michel Lespinasse <walken@google.com>]

migration_entry_wait() needs to take the rcu read lock so that page counts
can be guaranteed to be stable after one rcu grace period.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
 mm/migrate.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 666e4e6..6f3b5db 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -193,6 +193,7 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 	struct page *page;
 
 	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
+	rcu_read_lock();
 	pte = *ptep;
 	if (!is_swap_pte(pte))
 		goto out;
@@ -212,11 +213,13 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,
 	 */
 	if (!get_page_unless_zero(page))
 		goto out;
+	rcu_read_unlock();
 	pte_unmap_unlock(ptep, ptl);
 	wait_on_page_locked(page);
 	put_page(page);
 	return;
 out:
+	rcu_read_unlock();
 	pte_unmap_unlock(ptep, ptl);
 }
 
-- 
1.7.3.1