[PATCH] exfat: optimize allocation bitmap loading time

[Namjae Jeon <linkinjeon@xxxxxxxxxx>]

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@xxxxxxxxxxx>
Signed-off-by: Namjae Jeon <linkinjeon@xxxxxxxxxx>
---
 fs/exfat/balloc.c   | 12 +++++++++++-
 fs/exfat/dir.c      |  1 -
 fs/exfat/exfat_fs.h |  1 +
 3 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/fs/exfat/balloc.c b/fs/exfat/balloc.c
index cc01556c9d9b..c40b73701941 100644
--- a/fs/exfat/balloc.c
+++ b/fs/exfat/balloc.c
@@ -30,9 +30,11 @@ 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, need_map_size;
+	unsigned int i, j, need_map_size;
 	sector_t sector;
+	unsigned int max_ra_count = EXFAT_MAX_RA_SIZE >> sb->s_blocksize_bits;
 
 	sbi->map_clu = le32_to_cpu(ep->dentry.bitmap.start_clu);
 	map_size = le64_to_cpu(ep->dentry.bitmap.size);
@@ -57,6 +59,14 @@ static int exfat_allocate_bitmap(struct super_block *sb,
 
 	sector = exfat_cluster_to_sector(sbi, sbi->map_clu);
 	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]) {
 			/* release all buffers and free vol_amap */
diff --git a/fs/exfat/dir.c b/fs/exfat/dir.c
index ee060e26f51d..e7a8550c0346 100644
--- a/fs/exfat/dir.c
+++ b/fs/exfat/dir.c
@@ -616,7 +616,6 @@ static int exfat_find_location(struct super_block *sb, struct exfat_chain *p_dir
 	return 0;
 }
 
-#define EXFAT_MAX_RA_SIZE     (128*1024)
 static int exfat_dir_readahead(struct super_block *sb, sector_t sec)
 {
 	struct exfat_sb_info *sbi = EXFAT_SB(sb);
diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index f8ead4d47ef0..d1792d5c9eed 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -13,6 +13,7 @@
 #include <uapi/linux/exfat.h>
 
 #define EXFAT_ROOT_INO		1
+#define EXFAT_MAX_RA_SIZE     (128*1024)
 
 /*
  * exfat error flags
-- 
2.25.1