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>

1 files changed, 9 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 74f95ae0510b..745b3482e6c2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1394,8 +1394,15 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
          */
         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;