swap: change block allocation algorithm for SSD

I'm using a fast SSD to do swap.  scan_swap_map() sometimes uses up to
20~30% CPU time (when cluster is hard to find, the CPU time can be up to
80%), which becomes a bottleneck.  scan_swap_map() scans a byte array to
search a 256 page cluster, which is very slow.

Here I introduced a simple algorithm to search cluster.  Since we only
care about 256 pages cluster, we can just use a counter to track if a
cluster is free.  Every 256 pages use one int to store the counter.  If
the counter of a cluster is 0, the cluster is free.  All free clusters
will be added to a list, so searching cluster is very efficient.  With
this, scap_swap_map() overhead disappears.

This might help low end SD card swap too.  Because if the cluster is
aligned, SD firmware can do flash erase more efficiently.

We only enable the algorithm for SSD.  Hard disk swap isn't fast enough
and has downside with the algorithm which might introduce regression (see
below).

The patch slightly changes which cluster is choosen.  It always adds free
cluster to list tail.  This can help wear leveling for low end SSD too.
And if no cluster found, the scan_swap_map() will do search from the end
of last cluster.  So if no cluster found, the scan_swap_map() will do
search from the end of last free cluster, which is random.  For SSD, this
isn't a problem at all.

Another downside is the cluster must be aligned to 256 pages, which will
reduce the chance to find a cluster.  I would expect this isn't a big
problem for SSD because of the non-seek penality.  (And this is the reason
I only enable the algorithm for SSD).
Signed-off-by: Shaohua Li <shli@fusionio.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kyungmin Park <kmpark@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -181,6 +181,23 @@ enum {
 #define COUNT_CONTINUED	0x80	/* See swap_map continuation for full count */
 #define SWAP_MAP_SHMEM	0xbf	/* Owned by shmem/tmpfs, in first swap_map */
 
+/*
+ * We use this to track usage of a cluster. A cluster is a block of swap disk
+ * space with SWAPFILE_CLUSTER pages long and naturally aligns in disk. All
+ * free clusters are organized into a list. We fetch an entry from the list to
+ * get a free cluster.
+ *
+ * The data field stores next cluster if the cluster is free or cluster usage
+ * counter otherwise. The flags field determines if a cluster is free. This is
+ * protected by swap_info_struct.lock.
+ */
+struct swap_cluster_info {
+	unsigned int data:24;
+	unsigned int flags:8;
+};
+#define CLUSTER_FLAG_FREE 1 /* This cluster is free */
+#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */
+
 /*
  * The in-memory structure used to track swap areas.
  */
@@ -191,6 +208,9 @@ struct swap_info_struct {
 	signed char	next;		/* next type on the swap list */
 	unsigned int	max;		/* extent of the swap_map */
 	unsigned char *swap_map;	/* vmalloc'ed array of usage counts */
+	struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
+	struct swap_cluster_info free_cluster_head; /* free cluster list head */
+	struct swap_cluster_info free_cluster_tail; /* free cluster list tail */
 	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 */