mm/page_owner: avoid null pointer dereference

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

Fixes: f86e4271978b ("mm: check the return value of lookup_page_ext for all call sites")
Link: http://lkml.kernel.org/r/1465249059-7883-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/mm/page_owner.c b/mm/page_owner.c
index c6cda3e36212..fedeba88c9cb 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -207,13 +207,15 @@ void __dump_page_owner(struct page *page)
                 .nr_entries = page_ext->nr_entries,
                 .entries = &page_ext->trace_entries[0],
         };
-        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");