exfat: optimize allocation bitmap loading time

Loading the allocation bitmap is very slow if user set the small cluster
size on large partition.

For optimizing it, This patch uses sb_breadahead() read the allocation
bitmap. It will improve the mount time.

The following is the result of about 4TB partition(2KB cluster size)
on my target.

without patch:
real 0m41.746s
user 0m0.011s
sys 0m0.000s

with patch:
real 0m2.525s
user 0m0.008s
sys 0m0.008s

Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: namjaejeon

fs/exfat/balloc.c

@@ -7,6 +7,7 @@
 #include <linux/slab.h>
 #include <linux/bitmap.h>
 #include <linux/buffer_head.h>
+#include <linux/backing-dev.h>
 
 #include "exfat_raw.h"
 #include "exfat_fs.h"
@@ -73,9 +74,11 @@ static int exfat_allocate_bitmap(struct super_block *sb,
 		struct exfat_dentry *ep)
 {
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
+	struct blk_plug plug;
 	long long map_size;
 	unsigned int i, j, need_map_size;
 	sector_t sector;
+	unsigned int max_ra_count;
 
 	sbi->map_clu = le32_to_cpu(ep->dentry.bitmap.start_clu);
 	map_size = le64_to_cpu(ep->dentry.bitmap.size);
@@ -99,7 +102,17 @@ static int exfat_allocate_bitmap(struct super_block *sb,
 		return -ENOMEM;
 
 	sector = exfat_cluster_to_sector(sbi, sbi->map_clu);
+	max_ra_count = min(sb->s_bdi->ra_pages, sb->s_bdi->io_pages) <<
+		(PAGE_SHIFT - sb->s_blocksize_bits);
 	for (i = 0; i < sbi->map_sectors; i++) {
+		/* Trigger the next readahead in advance. */
+		if (0 == (i % max_ra_count)) {
+			blk_start_plug(&plug);
+			for (j = i; j < min(max_ra_count, sbi->map_sectors - i) + i; j++)
+				sb_breadahead(sb, sector + j);
+			blk_finish_plug(&plug);
+		}
+
 		sbi->vol_amap[i] = sb_bread(sb, sector + i);
 		if (!sbi->vol_amap[i])
 			goto err_out;