[PATCH v1 1/3] exfat: add block readahead in exfat_chain_cont_cluster

[Chi Zhiling <chizhiling@163.com>]

From: Chi Zhiling <chizhiling@kylinos.cn>

The conversion from NO_FAT_CHAIN format to FAT_CHAIN format occurs
when the file cannot allocate contiguous space. When the file to be
converted is very large, this process can take a long time.

This patch introduces simple readahead to read all the blocks in
advance, as these blocks are consecutive.

Test in an empty exfat filesystem:
dd if=/dev/zero of=/mnt/file bs=1M count=30k
dd if=/dev/zero of=/mnt/file2 bs=1M count=1
time cat /mnt/file2 >> /mnt/file

| cluster size | before patch | after patch |
| ------------ | ------------ | ----------- |
| 512          | 47.667s      | 4.316s      |
| 4k           | 6.436s       | 0.541s      |
| 32k          | 0.758s       | 0.071s      |
| 256k         | 0.117s       | 0.011s      |

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 fs/exfat/exfat_fs.h |  9 +++++++--
 fs/exfat/fatent.c   | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index 2dbed5f8ec26..5a3cdf725846 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -10,6 +10,7 @@
 #include <linux/ratelimit.h>
 #include <linux/nls.h>
 #include <linux/blkdev.h>
+#include <linux/backing-dev.h>
 #include <uapi/linux/exfat.h>
 
 #define EXFAT_ROOT_INO		1
@@ -79,6 +80,10 @@ enum {
 #define EXFAT_HINT_NONE		-1
 #define EXFAT_MIN_SUBDIR	2
 
+#define EXFAT_BLK_RA_SIZE(sb)		\
+(min((sb)->s_bdi->ra_pages, (sb)->s_bdi->io_pages) \
+	 << (PAGE_SHIFT - sb->s_blocksize_bits))
+
 /*
  * helpers for cluster size to byte conversion.
  */
@@ -117,9 +122,9 @@ enum {
 #define FAT_ENT_SIZE (4)
 #define FAT_ENT_SIZE_BITS (2)
 #define FAT_ENT_OFFSET_SECTOR(sb, loc) (EXFAT_SB(sb)->FAT1_start_sector + \
-	(((u64)loc << FAT_ENT_SIZE_BITS) >> sb->s_blocksize_bits))
+	(((u64)(loc) << FAT_ENT_SIZE_BITS) >> sb->s_blocksize_bits))
 #define FAT_ENT_OFFSET_BYTE_IN_SECTOR(sb, loc)	\
-	((loc << FAT_ENT_SIZE_BITS) & (sb->s_blocksize - 1))
+	(((loc) << FAT_ENT_SIZE_BITS) & (sb->s_blocksize - 1))
 
 /*
  * helpers for bitmap.
diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
index 71ee16479c43..0c17621587d5 100644
--- a/fs/exfat/fatent.c
+++ b/fs/exfat/fatent.c
@@ -142,13 +142,51 @@ int exfat_ent_get(struct super_block *sb, unsigned int loc,
 	return -EIO;
 }
 
+static int exfat_blk_readahead(struct super_block *sb, sector_t sec,
+		sector_t *ra, blkcnt_t *ra_cnt, sector_t end)
+{
+	struct blk_plug plug;
+
+	if (sec < *ra)
+		return 0;
+
+	*ra += *ra_cnt;
+
+	/* No blocks left (or only the last block), skip readahead. */
+	if (*ra >= end)
+		return 0;
+
+	*ra_cnt = min(end - *ra + 1, EXFAT_BLK_RA_SIZE(sb));
+	if (*ra_cnt == 0) {
+		/* Move 'ra' to the end to disable readahead. */
+		*ra = end;
+		return 0;
+	}
+
+	blk_start_plug(&plug);
+	for (unsigned int i = 0; i < *ra_cnt; i++)
+		sb_breadahead(sb, *ra + i);
+	blk_finish_plug(&plug);
+	return 0;
+}
+
 int exfat_chain_cont_cluster(struct super_block *sb, unsigned int chain,
 		unsigned int len)
 {
+	sector_t sec, end, ra;
+	blkcnt_t ra_cnt;
+
 	if (!len)
 		return 0;
 
+	ra_cnt = 0;
+	ra = FAT_ENT_OFFSET_SECTOR(sb, chain);
+	end = FAT_ENT_OFFSET_SECTOR(sb, chain + len - 1);
+
 	while (len > 1) {
+		sec = FAT_ENT_OFFSET_SECTOR(sb, chain);
+		exfat_blk_readahead(sb, sec, &ra, &ra_cnt, end);
+
 		if (exfat_ent_set(sb, chain, chain + 1))
 			return -EIO;
 		chain++;
-- 
2.43.0