mm, swap: avoid lock swap_avail_lock when held cluster lock

Cluster lock is used to protect the swap_cluster_info and corresponding
elements in swap_info_struct->swap_map[].  But it is found that now in
scan_swap_map_slots(), swap_avail_lock may be acquired when cluster lock
is held.  This does no good except making the locking more complex and
improving the potential locking contention, because the
swap_info_struct->lock is used to protect the data structure operated in
the code already.  Fix this via moving the corresponding operations in
scan_swap_map_slots() out of cluster lock.

Link: http://lkml.kernel.org/r/20170317064635.12792-3-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Acked-by: Tim Chen <tim.c.chen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -672,6 +672,9 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 		else
 			goto done;
 	}
+	si->swap_map[offset] = usage;
+	inc_cluster_info_page(si, si->cluster_info, offset);
+	unlock_cluster(ci);
 
 	if (offset == si->lowest_bit)
 		si->lowest_bit++;
@@ -685,9 +688,6 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
 		plist_del(&si->avail_list, &swap_avail_head);
 		spin_unlock(&swap_avail_lock);
 	}
-	si->swap_map[offset] = usage;
-	inc_cluster_info_page(si, si->cluster_info, offset);
-	unlock_cluster(ci);
 	si->cluster_next = offset + 1;
 	slots[n_ret++] = swp_entry(si->type, offset);