exfat: optimize exfat_chain_cont_cluster with cached buffer heads

When converting files from NO_FAT_CHAIN to FAT_CHAIN format, profiling
reveals significant time spent in mark_buffer_dirty() and exfat_mirror_bh()
operations. This overhead occurs because each FAT entry modification
triggers a full block dirty marking and mirroring operation.

For consecutive clusters that reside in the same block, optimize by caching
the buffer head and performing dirty marking only once at the end of the
block's modifications.

Performance improvements for converting a 30GB file:

| Cluster Size | Before Patch | After Patch | Speedup |
|--------------|--------------|-------------|---------|
| 512 bytes    | 4.243s       | 1.866s      | 2.27x   |
| 4KB          | 0.863s       | 0.236s      | 3.66x   |
| 32KB         | 0.069s       | 0.034s      | 2.03x   |
| 256KB        | 0.012s       | 0.006s      | 2.00x   |

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: Chi Zhiling

fs/exfat/fatent.c

@@ -32,6 +32,17 @@ static int exfat_mirror_bh(struct super_block *sb, struct buffer_head *bh)
 	return err;
 }
 
+static int exfat_end_bh(struct super_block *sb, struct buffer_head *bh)
+{
+	int err;
+
+	exfat_update_bh(bh, sb->s_flags & SB_SYNCHRONOUS);
+	err = exfat_mirror_bh(sb, bh);
+	brelse(bh);
+
+	return err;
+}
+
 static int __exfat_ent_get(struct super_block *sb, unsigned int loc,
 		unsigned int *content, struct buffer_head **last)
 {
@@ -62,29 +73,40 @@ static int __exfat_ent_get(struct super_block *sb, unsigned int loc,
 	return 0;
 }
 
-int exfat_ent_set(struct super_block *sb, unsigned int loc,
-		unsigned int content)
+static int __exfat_ent_set(struct super_block *sb, unsigned int loc,
+		unsigned int content, struct buffer_head **cache)
 {
-	unsigned int off;
 	sector_t sec;
 	__le32 *fat_entry;
-	struct buffer_head *bh;
+	struct buffer_head *bh = cache ? *cache : NULL;
+	unsigned int off;
 
 	sec = FAT_ENT_OFFSET_SECTOR(sb, loc);
 	off = FAT_ENT_OFFSET_BYTE_IN_SECTOR(sb, loc);
 
-	bh = sb_bread(sb, sec);
-	if (!bh)
-		return -EIO;
+	if (!bh || bh->b_blocknr != sec || !buffer_uptodate(bh)) {
+		if (bh)
+			exfat_end_bh(sb, bh);
+		bh = sb_bread(sb, sec);
+		if (cache)
+			*cache = bh;
+		if (unlikely(!bh))
+			return -EIO;
+	}
 
 	fat_entry = (__le32 *)&(bh->b_data[off]);
 	*fat_entry = cpu_to_le32(content);
-	exfat_update_bh(bh, sb->s_flags & SB_SYNCHRONOUS);
-	exfat_mirror_bh(sb, bh);
-	brelse(bh);
+	if (!cache)
+		exfat_end_bh(sb, bh);
 	return 0;
 }
 
+int exfat_ent_set(struct super_block *sb, unsigned int loc,
+		unsigned int content)
+{
+	return __exfat_ent_set(sb, loc, content, NULL);
+}
+
 /*
  * Caller must release the buffer_head if no error return.
  */
@@ -170,6 +192,7 @@ int exfat_blk_readahead(struct super_block *sb, sector_t sec,
 int exfat_chain_cont_cluster(struct super_block *sb, unsigned int chain,
 		unsigned int len)
 {
+	struct buffer_head *bh = NULL;
 	sector_t sec, end, ra;
 	blkcnt_t ra_cnt = 0;
 
@@ -183,14 +206,16 @@ int exfat_chain_cont_cluster(struct super_block *sb, unsigned int chain,
 		sec = FAT_ENT_OFFSET_SECTOR(sb, chain);
 		exfat_blk_readahead(sb, sec, &ra, &ra_cnt, end);
 
-		if (exfat_ent_set(sb, chain, chain + 1))
+		if (__exfat_ent_set(sb, chain, chain + 1, &bh))
 			return -EIO;
 		chain++;
 		len--;
 	}
 
-	if (exfat_ent_set(sb, chain, EXFAT_EOF_CLUSTER))
+	if (__exfat_ent_set(sb, chain, EXFAT_EOF_CLUSTER, &bh))
 		return -EIO;
+
+	exfat_end_bh(sb, bh);
 	return 0;
 }