swap_info: reorder its fields

Reorder (and comment) the fields of swap_info_struct, to make better
use of its cachelines: it's good for swap_duplicate() in particular
if unsigned int max and swap_map are near the start.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -167,21 +167,21 @@ struct swap_info_struct {
 	signed short	prio;		/* swap priority of this type */
 	signed char	type;		/* strange name for an index */
 	signed char	next;		/* next type on the swap list */
-	struct file *swap_file;
-	struct block_device *bdev;
-	struct swap_extent first_swap_extent;
-	struct swap_extent *curr_swap_extent;
-	unsigned char *swap_map;
-	unsigned int lowest_bit;
-	unsigned int highest_bit;
+	unsigned int	max;		/* extent of the swap_map */
+	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
+	unsigned int lowest_bit;	/* index of first free in swap_map */
+	unsigned int highest_bit;	/* index of last free in swap_map */
+	unsigned int pages;		/* total of usable pages of swap */
+	unsigned int inuse_pages;	/* number of those currently in use */
+	unsigned int cluster_next;	/* likely index for next allocation */
+	unsigned int cluster_nr;	/* countdown to next cluster search */
 	unsigned int lowest_alloc;	/* while preparing discard cluster */
 	unsigned int highest_alloc;	/* while preparing discard cluster */
-	unsigned int cluster_next;
-	unsigned int cluster_nr;
-	unsigned int pages;
-	unsigned int max;
-	unsigned int inuse_pages;
-	unsigned int old_block_size;
+	struct swap_extent *curr_swap_extent;
+	struct swap_extent first_swap_extent;
+	struct block_device *bdev;	/* swap device or bdev of swap file */
+	struct file *swap_file;		/* seldom referenced */
+	unsigned int old_block_size;	/* seldom referenced */
 };
 
 struct swap_list_t {