[PATCH v3 7/9] mm/page_owner: avoid null pointer dereference

[js1304]

From: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>

We have dereferenced page_ext before checking it. Lets check it first
and then used it.

Link: http://lkml.kernel.org/r/1465249059-7883-1-git-send-email-sudipm.mukherjee@xxxxxxxxx
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@xxxxxxxxxxxxxxx>
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/page_owner.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index dc92241..ec6dc18 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -295,13 +295,15 @@ void __dump_page_owner(struct page *page)
 		.skip = 0
 	};
 	depot_stack_handle_t handle;
-	gfp_t gfp_mask = page_ext->gfp_mask;
-	int mt = gfpflags_to_migratetype(gfp_mask);
+	gfp_t gfp_mask;
+	int mt;
 
 	if (unlikely(!page_ext)) {
 		pr_alert("There is not page extension available.\n");
 		return;
 	}
+	gfp_mask = page_ext->gfp_mask;
+	mt = gfpflags_to_migratetype(gfp_mask);
 
 	if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags)) {
 		pr_alert("page_owner info is not active (free page?)\n");
-- 
1.9.1