sys_swapon: do not depend on "type" after allocation

Within sys_swapon, after the swap_info entry has been allocated, we
always have type == p->type and swap_info[type] == p. Use this fact to
reduce the dependency on the "type" local variable within the function,
as a preparation to move the allocation of the swap_info entry to a
separate function.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Tested-by: Eric B Munson <emunson@mgebm.net>
Acked-by: Eric B Munson <emunson@mgebm.net>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -1927,7 +1927,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	for (i = 0; i < nr_swapfiles; i++) {
 		struct swap_info_struct *q = swap_info[i];
 
-		if (i == type || !q->swap_file)
+		if (q == p || !q->swap_file)
 			continue;
 		if (mapping == q->swap_file->f_mapping)
 			goto bad_swap;
@@ -2062,7 +2062,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 	}
 
-	error = swap_cgroup_swapon(type, maxpages);
+	error = swap_cgroup_swapon(p->type, maxpages);
 	if (error)
 		goto bad_swap;
 
@@ -2120,9 +2120,9 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	}
 	p->next = i;
 	if (prev < 0)
-		swap_list.head = swap_list.next = type;
+		swap_list.head = swap_list.next = p->type;
 	else
-		swap_info[prev]->next = type;
+		swap_info[prev]->next = p->type;
 	spin_unlock(&swap_lock);
 	mutex_unlock(&swapon_mutex);
 	atomic_inc(&proc_poll_event);
@@ -2136,7 +2136,7 @@ bad_swap:
 		blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);
 	}
 	destroy_swap_extents(p);
-	swap_cgroup_swapoff(type);
+	swap_cgroup_swapoff(p->type);
 bad_swap_2:
 	spin_lock(&swap_lock);
 	p->swap_file = NULL;