cow_user_page: fix page alignment
High Dickins points out that the user virtual address passed to the page
fault handler isn't necessarily page-aligned.
Also, add a comment on why the copy could fail for the user address case.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
diff --git a/mm/memory.c b/mm/memory.c
index 74f95ae..745b348 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1394,8 +1394,15 @@
*/
if (unlikely(!src)) {
void *kaddr = kmap_atomic(dst, KM_USER0);
- unsigned long left = __copy_from_user_inatomic(kaddr, (void __user *)va, PAGE_SIZE);
- if (left)
+ void __user *uaddr = (void __user *)(va & PAGE_MASK);
+
+ /*
+ * This really shouldn't fail, because the page is there
+ * in the page tables. But it might just be unreadable,
+ * in which case we just give up and fill the result with
+ * zeroes.
+ */
+ if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
memset(kaddr, 0, PAGE_SIZE);
kunmap_atomic(kaddr, KM_USER0);
return;
