[PATCH] md: improve 'hash' code in linear.c

The hashtable that linear uses to find the right device stores
two pointers for every entry.

The second is always one of:
   The first plus 1
   NULL
When NULL, it is never accessed, so any value can be stored.

Thus it could always be "first plus 1", and so we don't need to store
it as it is trivial to calculate.

This patch halves the size of this table, which results in some simpler
code as well.

Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/drivers/md/linear.c b/drivers/md/linear.c
index 7356511443617d64165368ce277c756f23acf702..161e9aa872917da3cc3bb2cc4719850317e7b9e1 100644 (file)
--- a/drivers/md/linear.c
+++ b/drivers/md/linear.c
@@ -31,7 +31,7 @@
  */
 static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
 {
-       struct linear_hash *hash;
+       dev_info_t *hash;
        linear_conf_t *conf = mddev_to_conf(mddev);
        sector_t block = sector >> 1;
 
@@ -39,12 +39,11 @@ static inline dev_info_t *which_dev(mddev_t *mddev, sector_t sector)
         * sector_div(a,b) returns the remainer and sets a to a/b
         */
        (void)sector_div(block, conf->smallest->size);
-       hash = conf->hash_table + block;
+       hash = conf->hash_table[block];
 
-       if ((sector>>1) >= (hash->dev0->size + hash->dev0->offset))
-               return hash->dev1;
-       else
-               return hash->dev0;
+       while ((sector>>1) >= (hash->size + hash->offset))
+               hash++;
+       return hash;
 }
 
 /**
@@ -114,7 +113,7 @@ static int linear_issue_flush(request_queue_t *q, struct gendisk *disk,
 static int linear_run (mddev_t *mddev)
 {
        linear_conf_t *conf;
-       struct linear_hash *table;
+       dev_info_t **table;
        mdk_rdev_t *rdev;
        int i, nb_zone, cnt;
        sector_t start;
@@ -184,7 +183,7 @@ static int linear_run (mddev_t *mddev)
                nb_zone = conf->nr_zones = sz + (round ? 1 : 0);
        }
                        
-       conf->hash_table = kmalloc (sizeof (struct linear_hash) * nb_zone,
+       conf->hash_table = kmalloc (sizeof (dev_info_t*) * nb_zone,
                                        GFP_KERNEL);
        if (!conf->hash_table)
                goto out;
@@ -198,9 +197,6 @@ static int linear_run (mddev_t *mddev)
        for (i = 0; i < cnt; i++) {
                dev_info_t *disk = conf->disks + i;
 
-               if (start > curr_offset)
-                       table[-1].dev1 = disk;
-
                disk->offset = curr_offset;
                curr_offset += disk->size;
 
@@ -208,10 +204,8 @@ static int linear_run (mddev_t *mddev)
                 * 'start' is the start of table
                 */
                while (start < curr_offset) {
-                       table->dev0 = disk;
-                       table->dev1 = NULL;
+                       *table++ = disk;
                        start += conf->smallest->size;
-                       table++;
                }
        }
        if (table-conf->hash_table != nb_zone)
@@ -255,13 +249,6 @@ static int linear_make_request (request_queue_t *q, struct bio *bio)
 
        tmp_dev = which_dev(mddev, bio->bi_sector);
        block = bio->bi_sector >> 1;
-  
-       if (unlikely(!tmp_dev)) {
-               printk("linear_make_request: hash->dev1==NULL for block %llu\n",
-                       (unsigned long long)block);
-               bio_io_error(bio, bio->bi_size);
-               return 0;
-       }
     
        if (unlikely(block >= (tmp_dev->size + tmp_dev->offset)
                     || block < tmp_dev->offset)) {
@@ -306,17 +293,20 @@ static void linear_status (struct seq_file *seq, mddev_t *mddev)
 #ifdef MD_DEBUG
        int j;
        linear_conf_t *conf = mddev_to_conf(mddev);
+       sector_t s = 0;
   
        seq_printf(seq, "      ");
        for (j = 0; j < conf->nr_zones; j++)
        {
                char b[BDEVNAME_SIZE];
+               s += conf->smallest_size;
                seq_printf(seq, "[%s",
-                          bdevname(conf->hash_table[j].dev0->rdev->bdev,b));
+                          bdevname(conf->hash_table[j][0].rdev->bdev,b));
 
-               if (conf->hash_table[j].dev1)
+               while (s > conf->hash_table[j][0].offset +
+                          conf->hash_table[j][0].size)
                        seq_printf(seq, "/%s] ",
-                                  bdevname(conf->hash_table[j].dev1->rdev->bdev,b));
+                                  bdevname(conf->hash_table[j][1].rdev->bdev,b));
                else
                        seq_printf(seq, "] ");
        }

diff --git a/include/linux/raid/linear.h b/include/linux/raid/linear.h
index e951b2bb9cdf12434730fbd8e2347afb7f71c09c..e04c4fe45b533303e1acc30f4d64c8a3cf987d36 100644 (file)
--- a/include/linux/raid/linear.h
+++ b/include/linux/raid/linear.h
@@ -11,14 +11,9 @@ struct dev_info {
 
 typedef struct dev_info dev_info_t;
 
-struct linear_hash
-{
-       dev_info_t *dev0, *dev1;
-};
-
 struct linear_private_data
 {
-       struct linear_hash      *hash_table;
+       dev_info_t              **hash_table;
        dev_info_t              *smallest;
        int                     nr_zones;
        dev_info_t              disks[0];