rmap: fix NULL-pointer dereference on THP munlocking

[mirror_ubuntu-artful-kernel.git] / mm / rmap.c

diff --git a/mm/rmap.c b/mm/rmap.c
index 2da487d6cea83b4f51db93bcbd05feaad31b927c..250635dc47b77badb2ad3ea86f6135535b0b6418 100644 (file)
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1316,12 +1316,6 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
        }
 
        while (page_vma_mapped_walk(&pvmw)) {
-               subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
-               address = pvmw.address;
-
-               /* Unexpected PMD-mapped THP? */
-               VM_BUG_ON_PAGE(!pvmw.pte, page);
-
                /*
                 * If the page is mlock()d, we cannot swap it out.
                 * If it's recently referenced (perhaps page_referenced
@@ -1345,6 +1339,13 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                                continue;
                }
 
+               /* Unexpected PMD-mapped THP? */
+               VM_BUG_ON_PAGE(!pvmw.pte, page);
+
+               subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
+               address = pvmw.address;
+
+
                if (!(flags & TTU_IGNORE_ACCESS)) {
                        if (ptep_clear_flush_young_notify(vma, address,
                                                pvmw.pte)) {