PATCH do_wp_page: fix regression with execute in place

[Carsten Otte <cotte@de.ibm.com>]

This patch fixes do_wp_page for VM_MIXEDMAP mappings. In case
pfn_valid returns 0 for a pfn at the beginning of do_wp_page,
and the mapping is not shared writable, the code branches to
label gotten with old_page == NULL. In case the vma is locked
(vma->vm_flags & VM_LOCKED), lock_page, clear_page_mlock, and
unlock_page try to access old_page.
This patch checks, whether old_page is valid before it is
dereferenced.
The regression was introduced with git commit b291f000

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
---
diff --git a/mm/memory.c b/mm/memory.c
index 22bfa7a..baa999e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1999,7 +1999,7 @@ gotten:
         * Don't let another task, with possibly unlocked vma,
         * keep the mlocked page.
         */
-       if (vma->vm_flags & VM_LOCKED) {
+       if ((vma->vm_flags & VM_LOCKED) && old_page) {
                lock_page(old_page);    /* for LRU manipulation */
                clear_page_mlock(old_page);
                unlock_page(old_page);

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