thp: change_huge_pmd(): make sure we don't try to make a page writable

mprotect core never tries to make page writable using change_huge_pmd().
Let's add an assert that the assumption is true.  It's important to be
sure we will not make huge zero page writable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a959b3a4ddd5471dc2e73609917bfa7acd0fc705..7742fb36eb4dd52e2d422eb3f50421b6a0c15928 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1273,6 +1273,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
                pmd_t entry;
                entry = pmdp_get_and_clear(mm, addr, pmd);
                entry = pmd_modify(entry, newprot);
+               BUG_ON(pmd_write(entry));
                set_pmd_at(mm, addr, pmd, entry);
                spin_unlock(&vma->vm_mm->page_table_lock);
                ret = 1;