[XFS] Revert the recent hashing change, performance seemed to go way down in

certain benchmarks.  This is reverted to how it was, except the number
of hash buckets is larger than previously to attempt to account for the
workload Steve was originally targetting with that change.

SGI Modid: 2.5.x-xfs:slinx:140053a

fs/xfs/pagebuf/page_buf.c

@@ -156,37 +156,35 @@ struct pbstats pbstats;
  * Pagebuf hashing
  */
 
-/* This structure must be a power of 2 long for the hash to work */
+#define NBITS	8
+#define NHASH	(1<<NBITS)
+
 typedef struct {
 	struct list_head	pb_hash;
 	int			pb_count;
 	spinlock_t		pb_hash_lock;
 } pb_hash_t;
 
-static pb_hash_t	*pbhash;
-static unsigned int	pb_hash_mask;
-static unsigned int	pb_hash_shift;
-static unsigned int	pb_order;
+STATIC pb_hash_t	pbhash[NHASH];
 #define pb_hash(pb)	&pbhash[pb->pb_hash_index]
 
-/*
- * This hash is the same one as used on the Linux buffer cache,
- * see fs/buffer.c
- */
-
-#define _hashfn(dev,block)      \
-        ((((dev)<<(pb_hash_shift - 6)) ^ ((dev)<<(pb_hash_shift - 9))) ^ \
-         (((block)<<(pb_hash_shift - 6)) ^ ((block) >> 13) ^ \
-          ((block) << (pb_hash_shift - 12))))
-
-static inline int
+STATIC int
 _bhash(
 	dev_t		dev,
 	loff_t		base)
 {
-	base >>= 9;
+	int		bit, hval;
 
-	return (_hashfn(dev, base) & pb_hash_mask);
+	base >>= 9;
+	/*
+	 * dev_t is 16 bits, loff_t is always 64 bits
+	 */
+	base ^= dev;
+	for (bit = hval = 0; base != 0 && bit < sizeof(base) * 8; bit += NBITS) {
+		hval ^= (int)base & (NHASH-1);
+		base >>= NBITS;
+	}
+	return hval;
 }
 
 /*
@@ -1869,39 +1867,7 @@ pagebuf_shaker(void)
 int __init
 pagebuf_init(void)
 {
-	int		order, mempages, i;
-	unsigned int	nr_hash;
-	extern int	xfs_physmem;
-
-	mempages = xfs_physmem >>= 16;
-	mempages *= sizeof(pb_hash_t);
-	for (order = 0; (1 << order) < mempages; order++)
-		;
-
-	if (order > 3) order = 3;	/* cap us at 2K buckets */
-
-	do {
-		unsigned long tmp;
-
-		nr_hash = (PAGE_SIZE << order) / sizeof(pb_hash_t);	
-		nr_hash = 1 << (ffs(nr_hash) - 1);
-		pb_hash_mask =  (nr_hash - 1);
-		tmp = nr_hash;
-		pb_hash_shift = 0;
-		while((tmp >>= 1UL) != 0UL)
-			pb_hash_shift++;
-
-		pbhash = (pb_hash_t *)
-			__get_free_pages(GFP_KERNEL, order);
-		pb_order = order;
-	} while (pbhash == NULL && --order > 0);
-	printk("pagebuf cache hash table entries: %d (order: %d, %ld bytes)\n",
-		nr_hash, order, (PAGE_SIZE << order));
-
-	for(i = 0; i < nr_hash; i++) {
-		spin_lock_init(&pbhash[i].pb_hash_lock);
-		INIT_LIST_HEAD(&pbhash[i].pb_hash);
-	} 
+	int			i;
 
 	pagebuf_table_header = register_sysctl_table(pagebuf_root_table, 1);
 
@@ -1919,6 +1885,11 @@ pagebuf_init(void)
 		return -ENOMEM;
 	}
 
+	for (i = 0; i < NHASH; i++) {
+		spin_lock_init(&pbhash[i].pb_hash_lock);
+		INIT_LIST_HEAD(&pbhash[i].pb_hash);
+	}
+
 #ifdef PAGEBUF_TRACE
 	pb_trace.buf = (pagebuf_trace_t *)kmalloc(
 			PB_TRACE_BUFSIZE * sizeof(pagebuf_trace_t), GFP_KERNEL);
@@ -1945,7 +1916,6 @@ pagebuf_terminate(void)
 
 	kmem_cache_destroy(pagebuf_cache);
 	kmem_shake_deregister(pagebuf_shaker);
-	free_pages((unsigned long)pbhash, pb_order);
 
 	unregister_sysctl_table(pagebuf_table_header);
 #ifdef	CONFIG_PROC_FS

fs/xfs/pagebuf/page_buf.h

@@ -215,8 +215,8 @@ typedef struct page_buf_s {
 	unsigned short		pb_error;	/* error code on I/O */
 	unsigned short		pb_page_count;	/* size of page array */
 	unsigned short		pb_offset;	/* page offset in first page */
-	unsigned short		pb_hash_index;	/* hash table index	*/
 	unsigned char		pb_locked;	/* page array is locked */
+	unsigned char		pb_hash_index;	/* hash table index	*/
 	struct page		**pb_pages;	/* array of page pointers */
 	struct page		*pb_page_array[PB_PAGES]; /* inline pages */
 #ifdef PAGEBUF_LOCK_TRACKING