[PATCH 1/2] f2fs: add lookup_mode mount option

[PATCH 1/2] f2fs: add lookup_mode mount option

[Daniel Lee]

For casefolded directories, f2fs may fall back to a linear search if
a hash-based lookup fails. This can cause severe performance
regressions.

While this behavior can be controlled by userspace tools (e.g. mkfs,
fsck) by setting an on-disk flag, a kernel-level solution is needed
to guarantee the lookup behavior regardless of the on-disk state.

This commit introduces the 'lookup_mode' mount option to provide this
kernel-side control.

The option accepts three values:
- perf: (Default) Enforces a hash-only lookup. The linear fallback
  is always disabled.
- compat: Enables the linear search fallback for compatibility with
  directory entries from older kernels.
- auto: Determines the mode based on the on-disk flag, preserving the
  userspace-based behavior.

Signed-off-by: Daniel Lee <chullee@google.com>
---
 Documentation/filesystems/f2fs.rst | 19 ++++++++++++++
 fs/f2fs/dir.c                      | 17 ++++++++++++-
 fs/f2fs/f2fs.h                     | 41 ++++++++++++++++++++++++++++++
 fs/f2fs/super.c                    | 20 +++++++++++++++
 4 files changed, 96 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index 440e4ae74e44..01c657ff7ae2 100644
--- a/Documentation/filesystems/f2fs.rst
+++ b/Documentation/filesystems/f2fs.rst
@@ -370,6 +370,25 @@ errors=%s		 Specify f2fs behavior on critical errors. This supports modes:
 			 ====================== =============== =============== ========
 nat_bits		 Enable nat_bits feature to enhance full/empty nat blocks access,
 			 by default it's disabled.
+lookup_mode=%s		 Control the directory lookup behavior for casefolded
+			 directories. This option has no effect on directories
+			 that do not have the casefold feature enabled.
+
+			 ================== ========================================
+			 Value		    Description
+			 ================== ========================================
+			 perf		    (Default) Enforces a hash-only lookup.
+					    The linear search fallback is always
+					    disabled, ignoring the on-disk flag.
+			 compat		    Enables the linear search fallback for
+					    compatibility with directory entries
+					    created by older kernel that used a
+					    different case-folding algorithm.
+					    This mode ignores the on-disk flag.
+			 auto		    F2FS determines the mode based on the
+					    on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL`
+					    flag.
+			 ================== ========================================
 ======================== ============================================================
 
 Debugfs Entries
diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index c36b3b22bfff..ba032d21a997 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -16,6 +16,21 @@
 #include "xattr.h"
 #include <trace/events/f2fs.h>
 
+static inline bool f2fs_should_fallback_to_linear(struct inode *dir)
+{
+	struct f2fs_sb_info *sbi = F2FS_I_SB(dir);
+
+	switch (f2fs_get_lookup_mode(sbi)) {
+	case LOOKUP_PERF:
+		return false;
+	case LOOKUP_COMPAT:
+		return true;
+	case LOOKUP_AUTO:
+		return !sb_no_casefold_compat_fallback(sbi->sb);
+	}
+	return false;
+}
+
 #if IS_ENABLED(CONFIG_UNICODE)
 extern struct kmem_cache *f2fs_cf_name_slab;
 #endif
@@ -366,7 +381,7 @@ struct f2fs_dir_entry *__f2fs_find_entry(struct inode *dir,
 
 out:
 #if IS_ENABLED(CONFIG_UNICODE)
-	if (!sb_no_casefold_compat_fallback(dir->i_sb) &&
+	if (f2fs_should_fallback_to_linear(dir) &&
 		IS_CASEFOLDED(dir) && !de && use_hash) {
 		use_hash = false;
 		goto start_find_entry;
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 9333a22b9a01..fed588f4fa3d 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -132,6 +132,12 @@ extern const char *f2fs_fault_name[FAULT_MAX];
  */
 #define F2FS_MOUNT_LAZYTIME		0x40000000
 
+enum f2fs_lookup_mode {
+	LOOKUP_PERF,
+	LOOKUP_COMPAT,
+	LOOKUP_AUTO,
+};
+
 #define F2FS_OPTION(sbi)	((sbi)->mount_opt)
 #define clear_opt(sbi, option)	(F2FS_OPTION(sbi).opt &= ~F2FS_MOUNT_##option)
 #define set_opt(sbi, option)	(F2FS_OPTION(sbi).opt |= F2FS_MOUNT_##option)
@@ -1355,6 +1361,8 @@ enum {
 	SBI_IS_RESIZEFS,			/* resizefs is in process */
 	SBI_IS_FREEZING,			/* freezefs is in process */
 	SBI_IS_WRITABLE,			/* remove ro mountoption transiently */
+	SBI_LOOKUP_COMPAT,			/* enable compat/auto lookup modes */
+	SBI_LOOKUP_AUTO,			/* enable auto lookup mode */
 	MAX_SBI_FLAG,
 };
 
@@ -4897,6 +4905,39 @@ static inline void f2fs_invalidate_internal_cache(struct f2fs_sb_info *sbi,
 	f2fs_invalidate_compress_pages_range(sbi, blkaddr, len);
 }
 
+/*
+ * The lookup mode is stored in two bits within sbi->s_flag:
+ *
+ * SBI_LOOKUP_COMPAT | SBI_LOOKUP_AUTO | Mode
+ * ------------------|-----------------|--------
+ *          0        |         0       | perf
+ *          1        |         0       | compat
+ *          1        |         1       | auto
+ *
+ */
+static inline enum f2fs_lookup_mode f2fs_get_lookup_mode(struct f2fs_sb_info *sbi)
+{
+	if (!is_sbi_flag_set(sbi, SBI_LOOKUP_COMPAT))
+		return LOOKUP_PERF;
+	if (is_sbi_flag_set(sbi, SBI_LOOKUP_AUTO))
+		return LOOKUP_AUTO;
+	return LOOKUP_COMPAT;
+}
+
+static inline void f2fs_set_lookup_mode(struct f2fs_sb_info *sbi,
+						enum f2fs_lookup_mode mode)
+{
+	clear_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
+	clear_sbi_flag(sbi, SBI_LOOKUP_AUTO);
+
+	if (mode == LOOKUP_COMPAT)
+		set_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
+	else if (mode == LOOKUP_AUTO) {
+		set_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
+		set_sbi_flag(sbi, SBI_LOOKUP_AUTO);
+	}
+}
+
 #define EFSBADCRC	EBADMSG		/* Bad CRC detected */
 #define EFSCORRUPTED	EUCLEAN		/* Filesystem is corrupted */
 
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index bbf1dad6843f..09cdd4c22e58 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -196,6 +196,7 @@ enum {
 	Opt_age_extent_cache,
 	Opt_errors,
 	Opt_nat_bits,
+	Opt_lookup_mode,
 	Opt_err,
 };
 
@@ -276,6 +277,7 @@ static match_table_t f2fs_tokens = {
 	{Opt_age_extent_cache, "age_extent_cache"},
 	{Opt_errors, "errors=%s"},
 	{Opt_nat_bits, "nat_bits"},
+	{Opt_lookup_mode, "lookup_mode=%s"},
 	{Opt_err, NULL},
 };
 
@@ -1317,6 +1319,22 @@ static int parse_options(struct f2fs_sb_info *sbi, char *options, bool is_remoun
 		case Opt_nat_bits:
 			set_opt(sbi, NAT_BITS);
 			break;
+		case Opt_lookup_mode:
+			name = match_strdup(&args[0]);
+			if (!name)
+				return -ENOMEM;
+			if (!strcmp(name, "perf")) {
+				f2fs_set_lookup_mode(sbi, LOOKUP_PERF);
+			} else if (!strcmp(name, "compat")) {
+				f2fs_set_lookup_mode(sbi, LOOKUP_COMPAT);
+			} else if (!strcmp(name, "auto")) {
+				f2fs_set_lookup_mode(sbi, LOOKUP_AUTO);
+			} else {
+				kfree(name);
+				return -EINVAL;
+			}
+			kfree(name);
+			break;
 		default:
 			f2fs_err(sbi, "Unrecognized mount option \"%s\" or missing value",
 				 p);
@@ -2220,6 +2238,8 @@ static void default_options(struct f2fs_sb_info *sbi, bool remount)
 #endif
 
 	f2fs_build_fault_attr(sbi, 0, 0, FAULT_ALL);
+
+	f2fs_set_lookup_mode(sbi, LOOKUP_PERF);
 }
 
 #ifdef CONFIG_QUOTA
-- 
2.50.1.565.gc32cd1483b-goog

[PATCH 2/2] f2fs: add sysfs entry for effective lookup mode

[Daniel Lee]

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>
---
 Documentation/ABI/testing/sysfs-fs-f2fs | 14 ++++++++++++++
 fs/f2fs/sysfs.c                         | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index bf03263b9f46..e847a50e7403 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -861,3 +861,17 @@ Description:	This is a read-only entry to show the value of sb.s_encoding_flags,
 		SB_ENC_STRICT_MODE_FL            0x00000001
 		SB_ENC_NO_COMPAT_FALLBACK_FL     0x00000002
 		============================     ==========
+
+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
+		  for casefolded directories.
diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index 75134d69a0bd..9897dd8564e0 100644
--- a/fs/f2fs/sysfs.c
+++ b/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_get_lookup_mode(sbi)) {
+	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, "perf\n");
+		return sysfs_emit(buf, "compat\n");
+	}
+	return 0;
+}
+
 static ssize_t mounted_time_sec_show(struct f2fs_attr *a,
 		struct f2fs_sb_info *sbi, char *buf)
 {
@@ -1166,6 +1182,7 @@ 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);
@@ -1282,6 +1299,7 @@ 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),
-- 
2.50.1.565.gc32cd1483b-goog

Re: [PATCH 1/2] f2fs: add lookup_mode mount option

[Chao Yu]

Daniel,

On 8/4/25 14:03, Daniel Lee wrote:
> For casefolded directories, f2fs may fall back to a linear search if
> a hash-based lookup fails. This can cause severe performance
> regressions.
> 
> While this behavior can be controlled by userspace tools (e.g. mkfs,
> fsck) by setting an on-disk flag, a kernel-level solution is needed
> to guarantee the lookup behavior regardless of the on-disk state.
> 
> This commit introduces the 'lookup_mode' mount option to provide this
> kernel-side control.
> 
> The option accepts three values:
> - perf: (Default) Enforces a hash-only lookup. The linear fallback
>   is always disabled.
> - compat: Enables the linear search fallback for compatibility with
>   directory entries from older kernels.
> - auto: Determines the mode based on the on-disk flag, preserving the
>   userspace-based behavior.
> 
> Signed-off-by: Daniel Lee <chullee@google.com>
> ---
>  Documentation/filesystems/f2fs.rst | 19 ++++++++++++++
>  fs/f2fs/dir.c                      | 17 ++++++++++++-
>  fs/f2fs/f2fs.h                     | 41 ++++++++++++++++++++++++++++++
>  fs/f2fs/super.c                    | 20 +++++++++++++++
>  4 files changed, 96 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
> index 440e4ae74e44..01c657ff7ae2 100644
> --- a/Documentation/filesystems/f2fs.rst
> +++ b/Documentation/filesystems/f2fs.rst
> @@ -370,6 +370,25 @@ errors=%s		 Specify f2fs behavior on critical errors. This supports modes:
>  			 ====================== =============== =============== ========
>  nat_bits		 Enable nat_bits feature to enhance full/empty nat blocks access,
>  			 by default it's disabled.
> +lookup_mode=%s		 Control the directory lookup behavior for casefolded
> +			 directories. This option has no effect on directories
> +			 that do not have the casefold feature enabled.
> +
> +			 ================== ========================================
> +			 Value		    Description
> +			 ================== ========================================
> +			 perf		    (Default) Enforces a hash-only lookup.
> +					    The linear search fallback is always
> +					    disabled, ignoring the on-disk flag.
> +			 compat		    Enables the linear search fallback for
> +					    compatibility with directory entries
> +					    created by older kernel that used a
> +					    different case-folding algorithm.
> +					    This mode ignores the on-disk flag.
> +			 auto		    F2FS determines the mode based on the
> +					    on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL`
> +					    flag.
> +			 ================== ========================================
>  ======================== ============================================================
>  
>  Debugfs Entries
> diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
> index c36b3b22bfff..ba032d21a997 100644
> --- a/fs/f2fs/dir.c
> +++ b/fs/f2fs/dir.c
> @@ -16,6 +16,21 @@
>  #include "xattr.h"
>  #include <trace/events/f2fs.h>
>  
> +static inline bool f2fs_should_fallback_to_linear(struct inode *dir)
> +{
> +	struct f2fs_sb_info *sbi = F2FS_I_SB(dir);
> +
> +	switch (f2fs_get_lookup_mode(sbi)) {
> +	case LOOKUP_PERF:
> +		return false;
> +	case LOOKUP_COMPAT:
> +		return true;
> +	case LOOKUP_AUTO:
> +		return !sb_no_casefold_compat_fallback(sbi->sb);
> +	}
> +	return false;
> +}
> +
>  #if IS_ENABLED(CONFIG_UNICODE)
>  extern struct kmem_cache *f2fs_cf_name_slab;
>  #endif
> @@ -366,7 +381,7 @@ struct f2fs_dir_entry *__f2fs_find_entry(struct inode *dir,
>  
>  out:
>  #if IS_ENABLED(CONFIG_UNICODE)
> -	if (!sb_no_casefold_compat_fallback(dir->i_sb) &&
> +	if (f2fs_should_fallback_to_linear(dir) &&
>  		IS_CASEFOLDED(dir) && !de && use_hash) {
>  		use_hash = false;
>  		goto start_find_entry;
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 9333a22b9a01..fed588f4fa3d 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -132,6 +132,12 @@ extern const char *f2fs_fault_name[FAULT_MAX];
>   */
>  #define F2FS_MOUNT_LAZYTIME		0x40000000
>  
> +enum f2fs_lookup_mode {
> +	LOOKUP_PERF,
> +	LOOKUP_COMPAT,
> +	LOOKUP_AUTO,
> +};

We're going to use new mount API, please check last code in dev-test branch,
and it's better to adapt code based on that.

> +
>  #define F2FS_OPTION(sbi)	((sbi)->mount_opt)
>  #define clear_opt(sbi, option)	(F2FS_OPTION(sbi).opt &= ~F2FS_MOUNT_##option)
>  #define set_opt(sbi, option)	(F2FS_OPTION(sbi).opt |= F2FS_MOUNT_##option)
> @@ -1355,6 +1361,8 @@ enum {
>  	SBI_IS_RESIZEFS,			/* resizefs is in process */
>  	SBI_IS_FREEZING,			/* freezefs is in process */
>  	SBI_IS_WRITABLE,			/* remove ro mountoption transiently */
> +	SBI_LOOKUP_COMPAT,			/* enable compat/auto lookup modes */
> +	SBI_LOOKUP_AUTO,			/* enable auto lookup mode */

Could you please add lookup_mode field in struct f2fs_mount_info to record lookup_mode
status like we did for other mount options? IIRC, in f2fs_remount(), we will record
all old mount options in sbi.mount_opt for later recovery if necessary.

>  	MAX_SBI_FLAG,
>  };
>  
> @@ -4897,6 +4905,39 @@ static inline void f2fs_invalidate_internal_cache(struct f2fs_sb_info *sbi,
>  	f2fs_invalidate_compress_pages_range(sbi, blkaddr, len);
>  }
>  
> +/*
> + * The lookup mode is stored in two bits within sbi->s_flag:
> + *
> + * SBI_LOOKUP_COMPAT | SBI_LOOKUP_AUTO | Mode
> + * ------------------|-----------------|--------
> + *          0        |         0       | perf
> + *          1        |         0       | compat
> + *          1        |         1       | auto
> + *
> + */
> +static inline enum f2fs_lookup_mode f2fs_get_lookup_mode(struct f2fs_sb_info *sbi)
> +{
> +	if (!is_sbi_flag_set(sbi, SBI_LOOKUP_COMPAT))
> +		return LOOKUP_PERF;
> +	if (is_sbi_flag_set(sbi, SBI_LOOKUP_AUTO))
> +		return LOOKUP_AUTO;
> +	return LOOKUP_COMPAT;
> +}
> +
> +static inline void f2fs_set_lookup_mode(struct f2fs_sb_info *sbi,
> +						enum f2fs_lookup_mode mode)
> +{
> +	clear_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
> +	clear_sbi_flag(sbi, SBI_LOOKUP_AUTO);
> +
> +	if (mode == LOOKUP_COMPAT)
> +		set_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
> +	else if (mode == LOOKUP_AUTO) {
> +		set_sbi_flag(sbi, SBI_LOOKUP_COMPAT);
> +		set_sbi_flag(sbi, SBI_LOOKUP_AUTO);
> +	}
> +}
> +
>  #define EFSBADCRC	EBADMSG		/* Bad CRC detected */
>  #define EFSCORRUPTED	EUCLEAN		/* Filesystem is corrupted */
>  
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index bbf1dad6843f..09cdd4c22e58 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -196,6 +196,7 @@ enum {
>  	Opt_age_extent_cache,
>  	Opt_errors,
>  	Opt_nat_bits,
> +	Opt_lookup_mode,
>  	Opt_err,
>  };
>  
> @@ -276,6 +277,7 @@ static match_table_t f2fs_tokens = {
>  	{Opt_age_extent_cache, "age_extent_cache"},
>  	{Opt_errors, "errors=%s"},
>  	{Opt_nat_bits, "nat_bits"},
> +	{Opt_lookup_mode, "lookup_mode=%s"},
>  	{Opt_err, NULL},
>  };
>  
> @@ -1317,6 +1319,22 @@ static int parse_options(struct f2fs_sb_info *sbi, char *options, bool is_remoun
>  		case Opt_nat_bits:
>  			set_opt(sbi, NAT_BITS);
>  			break;
> +		case Opt_lookup_mode:
> +			name = match_strdup(&args[0]);
> +			if (!name)
> +				return -ENOMEM;
> +			if (!strcmp(name, "perf")) {
> +				f2fs_set_lookup_mode(sbi, LOOKUP_PERF);
> +			} else if (!strcmp(name, "compat")) {
> +				f2fs_set_lookup_mode(sbi, LOOKUP_COMPAT);
> +			} else if (!strcmp(name, "auto")) {
> +				f2fs_set_lookup_mode(sbi, LOOKUP_AUTO);
> +			} else {
> +				kfree(name);
> +				return -EINVAL;
> +			}
> +			kfree(name);
> +			break;
>  		default:
>  			f2fs_err(sbi, "Unrecognized mount option \"%s\" or missing value",
>  				 p);
> @@ -2220,6 +2238,8 @@ static void default_options(struct f2fs_sb_info *sbi, bool remount)
>  #endif
>  
>  	f2fs_build_fault_attr(sbi, 0, 0, FAULT_ALL);
> +
> +	f2fs_set_lookup_mode(sbi, LOOKUP_PERF);

I guess we need to change f2fs_show_options() as well, right?

Thanks,

>  }
>  
>  #ifdef CONFIG_QUOTA

Re: [PATCH 2/2] f2fs: add sysfs entry for effective lookup mode

[Chao Yu]

On 8/4/25 14:03, Daniel Lee wrote:
> 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>
> ---
>  Documentation/ABI/testing/sysfs-fs-f2fs | 14 ++++++++++++++
>  fs/f2fs/sysfs.c                         | 18 ++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
> index bf03263b9f46..e847a50e7403 100644
> --- a/Documentation/ABI/testing/sysfs-fs-f2fs
> +++ b/Documentation/ABI/testing/sysfs-fs-f2fs
> @@ -861,3 +861,17 @@ Description:	This is a read-only entry to show the value of sb.s_encoding_flags,
>  		SB_ENC_STRICT_MODE_FL            0x00000001
>  		SB_ENC_NO_COMPAT_FALLBACK_FL     0x00000002
>  		============================     ==========
> +
> +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
> +		  for casefolded directories.

Any way to know whether "perf" or "compat" comes from auto mode or not? something
like "auto:perf", "auto:compat", maybe?

Thanks,

> diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
> index 75134d69a0bd..9897dd8564e0 100644
> --- a/fs/f2fs/sysfs.c
> +++ b/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_get_lookup_mode(sbi)) {
> +	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, "perf\n");
> +		return sysfs_emit(buf, "compat\n");
> +	}
> +	return 0;
> +}
> +
>  static ssize_t mounted_time_sec_show(struct f2fs_attr *a,
>  		struct f2fs_sb_info *sbi, char *buf)
>  {
> @@ -1166,6 +1182,7 @@ 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);
> @@ -1282,6 +1299,7 @@ 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),