erofs: support to readahead dirent blocks in erofs_readdir()

This patch supports to readahead more blocks in erofs_readdir(), it can
enhance readdir performance in large direcotry.

readdir test in a large directory which contains 12000 sub-files.

files_per_second
Before: 926385.54
After: 2380435.562

Meanwhile, let's introduces a new sysfs entry to control readahead
bytes to provide more flexible policy for readahead of readdir().
- location: /sys/fs/erofs/<disk>/dir_ra_bytes
- default value: 16384
- disable readahead: set the value to 0

Signed-off-by: Chao Yu <[e-mail]>
Reviewed-by: Gao Xiang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[ Gao Xiang: minor styling adjustment. ]
Signed-off-by: Gao Xiang <[e-mail]>

Documentation/ABI/testing/sysfs-fs-erofs

···
 		and multiple accelerators are separated by '\n'.
 		Supported accelerator(s): qat_deflate.
 		Disable all accelerators with an empty string (echo > accel).
+
+What:		/sys/fs/erofs/<disk>/dir_ra_bytes
+Date:		July 2025
+Contact:	"Chao Yu" <[e-mail]>
+Description:	Used to set or show readahead bytes during readdir(), by
+		default the value is 16384.
+
+		- 0: disable readahead.

fs/erofs/dir.c

···
 	struct inode *dir = file_inode(f);
 	struct erofs_buf buf = __EROFS_BUF_INITIALIZER;
 	struct super_block *sb = dir->i_sb;
+	struct file_ra_state *ra = &f->f_ra;
 	unsigned long bsz = sb->s_blocksize;
 	unsigned int ofs = erofs_blkoff(sb, ctx->pos);
+	pgoff_t ra_pages = DIV_ROUND_UP_POW2(
+			EROFS_I_SB(dir)->dir_ra_bytes, PAGE_SIZE);
+	pgoff_t nr_pages = DIV_ROUND_UP_POW2(dir->i_size, PAGE_SIZE);
 	int err = 0;
 	bool initial = true;
 
···
 		if (fatal_signal_pending(current)) {
 			err = -ERESTARTSYS;
 			break;
+		}
+
+		/* readahead blocks to enhance performance for large directories */
+		if (ra_pages) {
+			pgoff_t idx = DIV_ROUND_UP_POW2(ctx->pos, PAGE_SIZE);
+			pgoff_t pages = min(nr_pages - idx, ra_pages);
+
+			if (pages > 1 && !ra_has_index(ra, idx))
+				page_cache_sync_readahead(dir->i_mapping, ra,
+							  f, idx, pages);
 		}
 
 		de = erofs_bread(&buf, dbstart, true);

fs/erofs/internal.h

···
 	/* sysfs support */
 	struct kobject s_kobj;		/* /sys/fs/erofs/<devname> */
 	struct completion s_kobj_unregister;
+	erofs_off_t dir_ra_bytes;
 
 	/* fscache support */
 	struct fscache_volume *volume;
···
 /* bitlock definitions (arranged in reverse order) */
 #define EROFS_I_BL_XATTR_BIT	(BITS_PER_LONG - 1)
 #define EROFS_I_BL_Z_BIT	(BITS_PER_LONG - 2)
+
+/* default readahead size of directories */
+#define EROFS_DIR_RA_BYTES	16384
 
 struct erofs_inode {
 	erofs_nid_t nid;

fs/erofs/super.c

···
 	if (err)
 		return err;
 
+	sbi->dir_ra_bytes = EROFS_DIR_RA_BYTES;
 	erofs_info(sb, "mounted with root inode @ nid %llu.", sbi->root_nid);
 	return 0;
 }

fs/erofs/sysfs.c

···
 #ifdef CONFIG_EROFS_FS_ZIP_ACCEL
 EROFS_ATTR_FUNC(accel, 0644);
 #endif
+EROFS_ATTR_RW_UI(dir_ra_bytes, erofs_sb_info);
 
 static struct attribute *erofs_sb_attrs[] = {
 #ifdef CONFIG_EROFS_FS_ZIP
 	ATTR_LIST(sync_decompress),
 	ATTR_LIST(drop_caches),
 #endif
+	ATTR_LIST(dir_ra_bytes),
 	NULL,
 };
 ATTRIBUTE_GROUPS(erofs_sb);