gup: avoid multiple user access locking/unlocking in fault_in_{read/write}able

fault_in_readable() and fault_in_writeable() perform __get_user() and
__put_user() in a loop, implying multiple user access locking/unlocking.

To avoid that, use user access blocks.

Link: https://lkml.kernel.org/r/720dcf79314acca1a78fae56d478cc851952149d.1637084492.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -1672,21 +1672,22 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
 
 	if (unlikely(size == 0))
 		return 0;
+	if (!user_write_access_begin(uaddr, size))
+		return size;
 	if (!PAGE_ALIGNED(uaddr)) {
-		if (unlikely(__put_user(0, uaddr) != 0))
-			return size;
+		unsafe_put_user(0, uaddr, out);
 		uaddr = (char __user *)PAGE_ALIGN((unsigned long)uaddr);
 	}
 	end = (char __user *)PAGE_ALIGN((unsigned long)start + size);
 	if (unlikely(end < start))
 		end = NULL;
 	while (uaddr != end) {
-		if (unlikely(__put_user(0, uaddr) != 0))
-			goto out;
+		unsafe_put_user(0, uaddr, out);
 		uaddr += PAGE_SIZE;
 	}
 
 out:
+	user_write_access_end();
 	if (size > uaddr - start)
 		return size - (uaddr - start);
 	return 0;
@@ -1771,21 +1772,22 @@ size_t fault_in_readable(const char __user *uaddr, size_t size)
 
 	if (unlikely(size == 0))
 		return 0;
+	if (!user_read_access_begin(uaddr, size))
+		return size;
 	if (!PAGE_ALIGNED(uaddr)) {
-		if (unlikely(__get_user(c, uaddr) != 0))
-			return size;
+		unsafe_get_user(c, uaddr, out);
 		uaddr = (const char __user *)PAGE_ALIGN((unsigned long)uaddr);
 	}
 	end = (const char __user *)PAGE_ALIGN((unsigned long)start + size);
 	if (unlikely(end < start))
 		end = NULL;
 	while (uaddr != end) {
-		if (unlikely(__get_user(c, uaddr) != 0))
-			goto out;
+		unsafe_get_user(c, uaddr, out);
 		uaddr += PAGE_SIZE;
 	}
 
 out:
+	user_read_access_end();
 	(void)c;
 	if (size > uaddr - start)
 		return size - (uaddr - start);