fat: optimize fat_count_free_clusters()

On large partition, scanning the free clusters is very slow if users
doesn't use "usefree" option.

For optimizing it, this patch uses sb_breadahead() to read of FAT
sectors. On some user's 15GB partition, this patch improved it very
much (1min => 600ms).

The following is the result of 2GB partition on my machine.

without patch:
	root@devron (/)# time df -h > /dev/null

	real    0m1.202s
	user    0m0.000s
	sys     0m0.440s

with patch:
	root@devron (/)# time df -h > /dev/null

	real    0m0.378s
	user    0m0.012s
	sys     0m0.168s

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
index 2c1b73fb82ae..5fb366992b73 100644
--- a/fs/fat/fatent.c
+++ b/fs/fat/fatent.c
@@ -590,21 +590,49 @@ error:
 
 EXPORT_SYMBOL_GPL(fat_free_clusters);
 
+/* 128kb is the whole sectors for FAT12 and FAT16 */
+#define FAT_READA_SIZE          (128 * 1024)
+
+static void fat_ent_reada(struct super_block *sb, struct fat_entry *fatent,
+                          unsigned long reada_blocks)
+{
+        struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;
+        sector_t blocknr;
+        int i, offset;
+
+        ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);
+
+        for (i = 0; i < reada_blocks; i++)
+                sb_breadahead(sb, blocknr + i);
+}
+
 int fat_count_free_clusters(struct super_block *sb)
 {
         struct msdos_sb_info *sbi = MSDOS_SB(sb);
         struct fatent_operations *ops = sbi->fatent_ops;
         struct fat_entry fatent;
+        unsigned long reada_blocks, reada_mask, cur_block;
         int err = 0, free;
 
         lock_fat(sbi);
         if (sbi->free_clusters != -1)
                 goto out;
 
+        reada_blocks = FAT_READA_SIZE >> sb->s_blocksize_bits;
+        reada_mask = reada_blocks - 1;
+        cur_block = 0;
+
         free = 0;
         fatent_init(&fatent);
         fatent_set_entry(&fatent, FAT_START_ENT);
         while (fatent.entry < sbi->max_cluster) {
+                /* readahead of fat blocks */
+                if ((cur_block & reada_mask) == 0) {
+                        unsigned long rest = sbi->fat_length - cur_block;
+                        fat_ent_reada(sb, &fatent, min(reada_blocks, rest));
+                }
+                cur_block++;
+
                 err = fat_ent_read_block(sb, &fatent);
                 if (err)
                         goto out;