f2fs: add sysfs entry for effective lookup mode

This commit introduces a new read-only sysfs entry at
/sys/fs/f2fs/<device>/effective_lookup_mode.

This entry displays the actual directory lookup mode F2FS is
currently using. This is needed for debugging and verification,
as the behavior is determined by both on-disk flags and mount
options.

Signed-off-by: Daniel Lee <chullee@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Daniel Lee

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -883,3 +883,18 @@ Date:		June 2025
 Contact:	"Daeho Jeong" <daehojeong@google.com>
 Description:	Control GC algorithm for boost GC. 0: cost benefit, 1: greedy
 		Default: 1
+
+What:		/sys/fs/f2fs/<disk>/effective_lookup_mode
+Date:		August 2025
+Contact:	"Daniel Lee" <chullee@google.com>
+Description:
+		This is a read-only entry to show the effective directory lookup mode
+		F2FS is currently using for casefolded directories.
+		This considers both the "lookup_mode" mount option and the on-disk
+		encoding flag, SB_ENC_NO_COMPAT_FALLBACK_FL.
+
+		Possible values are:
+		- "perf": Hash-only lookup.
+		- "compat": Hash-based lookup with a linear search fallback enabled
+		- "auto:perf": lookup_mode is auto and fallback is disabled on-disk
+		- "auto:compat": lookup_mode is auto and fallback is enabled on-disk

fs/f2fs/sysfs.c

@@ -281,6 +281,22 @@ static ssize_t encoding_flags_show(struct f2fs_attr *a,
 		le16_to_cpu(F2FS_RAW_SUPER(sbi)->s_encoding_flags));
 }
 
+static ssize_t effective_lookup_mode_show(struct f2fs_attr *a,
+		struct f2fs_sb_info *sbi, char *buf)
+{
+	switch (F2FS_OPTION(sbi).lookup_mode) {
+	case LOOKUP_PERF:
+		return sysfs_emit(buf, "perf\n");
+	case LOOKUP_COMPAT:
+		return sysfs_emit(buf, "compat\n");
+	case LOOKUP_AUTO:
+		if (sb_no_casefold_compat_fallback(sbi->sb))
+			return sysfs_emit(buf, "auto:perf\n");
+		return sysfs_emit(buf, "auto:compat\n");
+	}
+	return 0;
+}
+
 static ssize_t mounted_time_sec_show(struct f2fs_attr *a,
 		struct f2fs_sb_info *sbi, char *buf)
 {
@@ -1211,6 +1227,7 @@ F2FS_GENERAL_RO_ATTR(current_reserved_blocks);
 F2FS_GENERAL_RO_ATTR(unusable);
 F2FS_GENERAL_RO_ATTR(encoding);
 F2FS_GENERAL_RO_ATTR(encoding_flags);
+F2FS_GENERAL_RO_ATTR(effective_lookup_mode);
 F2FS_GENERAL_RO_ATTR(mounted_time_sec);
 F2FS_GENERAL_RO_ATTR(main_blkaddr);
 F2FS_GENERAL_RO_ATTR(pending_discard);
@@ -1329,6 +1346,7 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(current_reserved_blocks),
 	ATTR_LIST(encoding),
 	ATTR_LIST(encoding_flags),
+	ATTR_LIST(effective_lookup_mode),
 	ATTR_LIST(mounted_time_sec),
 #ifdef CONFIG_F2FS_STAT_FS
 	ATTR_LIST(cp_foreground_calls),