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

kiryl · torvalds · commit cad7f613c4d0 · 2012-12-12T17:38:31.000-08:00
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 &lt;kirill.shutemov@linux.intel.com&gt;
Cc: Andrea Arcangeli &lt;aarcange@redhat.com&gt;
Cc: Andi Kleen &lt;ak@linux.intel.com&gt;
Cc: "H. Peter Anvin" &lt;hpa@linux.intel.com&gt;
Cc: Mel Gorman &lt;mel@csn.ul.ie&gt;
Cc: David Rientjes &lt;rientjes@google.com&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git 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;
