exfat: reduce FAT chain traversal

Before this commit, ->dir and ->entry of exfat_inode_info record the
first cluster of the parent directory and the directory entry index
starting from this cluster.

The directory entry set will be gotten during write-back-inode/rmdir/
unlink/rename. If the clusters of the parent directory are not
continuous, the FAT chain will be traversed from the first cluster of
the parent directory to find the cluster where ->entry is located.

After this commit, ->dir records the cluster where the first directory
entry in the directory entry set is located, and ->entry records the
directory entry index in the cluster, so that there is almost no need
to access the FAT when getting the directory entry set.

Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Aoyama Wataru <wataru.aoyama@sony.com>
Reviewed-by: Daniel Palmer <daniel.palmer@sony.com>
Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/exfat/dir.c

@@ -148,7 +148,8 @@ static int exfat_readdir(struct inode *inode, loff_t *cpos, struct exfat_dir_ent
 			ep = exfat_get_dentry(sb, &clu, i + 1, &bh);
 			if (!ep)
 				return -EIO;
-			dir_entry->entry = dentry;
+			dir_entry->entry = i;
+			dir_entry->dir = clu;
 			brelse(bh);
 
 			ei->hint_bmap.off = EXFAT_DEN_TO_CLU(dentry, sbi);
@@ -256,7 +257,7 @@ static int exfat_iterate(struct file *file, struct dir_context *ctx)
 	if (!nb->lfn[0])
 		goto end_of_dir;
 
-	i_pos = ((loff_t)ei->start_clu << 32) |	(de.entry & 0xffffffff);
+	i_pos = ((loff_t)de.dir.dir << 32) | (de.entry & 0xffffffff);
 	tmp = exfat_iget(sb, i_pos);
 	if (tmp) {
 		inum = tmp->i_ino;

fs/exfat/exfat_fs.h

@@ -204,7 +204,9 @@ struct exfat_entry_set_cache {
 #define IS_DYNAMIC_ES(es)	((es)->__bh != (es)->bh)
 
 struct exfat_dir_entry {
+	/* the cluster where file dentry is located */
 	struct exfat_chain dir;
+	/* the index of file dentry in ->dir */
 	int entry;
 	unsigned int type;
 	unsigned int start_clu;
@@ -290,7 +292,9 @@ struct exfat_sb_info {
  * EXFAT file system inode in-memory data
  */
 struct exfat_inode_info {
+	/* the cluster where file dentry is located */
 	struct exfat_chain dir;
+	/* the index of file dentry in ->dir */
 	int entry;
 	unsigned int type;
 	unsigned short attr;

fs/exfat/namei.c

@@ -288,8 +288,22 @@ static int exfat_check_max_dentries(struct inode *inode)
 	return 0;
 }
 
-/* find empty directory entry.
+/*
- * if there isn't any empty slot, expand cluster chain.
+ * Find an empty directory entry set.
+ *
+ * If there isn't any empty slot, expand cluster chain.
+ *
+ * in:
+ *   inode: inode of the parent directory
+ *   num_entries: specifies how many dentries in the empty directory entry set
+ *
+ * out:
+ *   p_dir: the cluster where the empty directory entry set is located
+ *   es: The found empty directory entry set
+ *
+ * return:
+ *   the directory entry index in p_dir is returned on succeeds
+ *   -error code is returned on failure
  */
 static int exfat_find_empty_entry(struct inode *inode,
 		struct exfat_chain *p_dir, int num_entries,
@@ -381,7 +395,10 @@ static int exfat_find_empty_entry(struct inode *inode,
 		inode->i_blocks += sbi->cluster_size >> 9;
 	}
 
-	return dentry;
+	p_dir->dir = exfat_sector_to_cluster(sbi, es->bh[0]->b_blocknr);
+	p_dir->size -= dentry / sbi->dentries_per_clu;
+
+	return dentry & (sbi->dentries_per_clu - 1);
 }
 
 /*
@@ -613,15 +630,16 @@ static int exfat_find(struct inode *dir, struct qstr *qname,
 	if (dentry < 0)
 		return dentry; /* -error value */
 
-	info->dir = cdir;
-	info->entry = dentry;
-	info->num_subdirs = 0;
-
 	/* adjust cdir to the optimized value */
 	cdir.dir = hint_opt.clu;
 	if (cdir.flags & ALLOC_NO_FAT_CHAIN)
 		cdir.size -= dentry / sbi->dentries_per_clu;
 	dentry = hint_opt.eidx;
+
+	info->dir = cdir;
+	info->entry = dentry;
+	info->num_subdirs = 0;
+
 	if (exfat_get_dentry_set(&es, sb, &cdir, dentry, ES_2_ENTRIES))
 		return -EIO;
 	ep = exfat_get_dentry_cached(&es, ES_IDX_FILE);