f2fs: add lookup_mode mount option

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>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

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

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_OPTION(sbi).lookup_mode) {
+	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 @@ start_find_entry:
 
 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;

fs/f2fs/f2fs.h

@@ -212,6 +212,7 @@ struct f2fs_mount_info {
 	int compress_mode;			/* compression mode */
 	unsigned char extensions[COMPRESS_EXT_NUM][F2FS_EXTENSION_LEN];	/* extensions */
 	unsigned char noextensions[COMPRESS_EXT_NUM][F2FS_EXTENSION_LEN]; /* extensions */
+	unsigned int lookup_mode;
 };
 
 #define F2FS_FEATURE_ENCRYPT			0x00000001
@@ -1475,6 +1476,12 @@ enum {
 	TOTAL_CALL = FOREGROUND,
 };
 
+enum f2fs_lookup_mode {
+	LOOKUP_PERF,
+	LOOKUP_COMPAT,
+	LOOKUP_AUTO,
+};
+
 static inline int f2fs_test_bit(unsigned int nr, char *addr);
 static inline void f2fs_set_bit(unsigned int nr, char *addr);
 static inline void f2fs_clear_bit(unsigned int nr, char *addr);

fs/f2fs/super.c

@@ -181,6 +181,7 @@ enum {
 	Opt_nat_bits,
 	Opt_jqfmt,
 	Opt_checkpoint,
+	Opt_lookup_mode,
 	Opt_err,
 };
 
@@ -244,6 +245,13 @@ static const struct constant_table f2fs_param_errors[] = {
 	{}
 };
 
+static const struct constant_table f2fs_param_lookup_mode[] = {
+	{"perf",	LOOKUP_PERF},
+	{"compat",	LOOKUP_COMPAT},
+	{"auto",	LOOKUP_AUTO},
+	{}
+};
+
 static const struct fs_parameter_spec f2fs_param_specs[] = {
 	fsparam_enum("background_gc", Opt_gc_background, f2fs_param_background_gc),
 	fsparam_flag("disable_roll_forward", Opt_disable_roll_forward),
@@ -300,6 +308,7 @@ static const struct fs_parameter_spec f2fs_param_specs[] = {
 	fsparam_enum("memory", Opt_memory_mode, f2fs_param_memory_mode),
 	fsparam_flag("age_extent_cache", Opt_age_extent_cache),
 	fsparam_enum("errors", Opt_errors, f2fs_param_errors),
+	fsparam_enum("lookup_mode", Opt_lookup_mode, f2fs_param_lookup_mode),
 	{}
 };
 
@@ -336,6 +345,7 @@ static match_table_t f2fs_checkpoint_tokens = {
 #define F2FS_SPEC_discard_unit			(1 << 21)
 #define F2FS_SPEC_memory_mode			(1 << 22)
 #define F2FS_SPEC_errors			(1 << 23)
+#define F2FS_SPEC_lookup_mode			(1 << 24)
 
 struct f2fs_fs_context {
 	struct f2fs_mount_info info;
@@ -1143,6 +1153,10 @@ static int f2fs_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_nat_bits:
 		ctx_set_opt(ctx, F2FS_MOUNT_NAT_BITS);
 		break;
+	case Opt_lookup_mode:
+		F2FS_CTX_INFO(ctx).lookup_mode = result.uint_32;
+		ctx->spec_mask |= F2FS_SPEC_lookup_mode;
+		break;
 	}
 	return 0;
 }
@@ -1652,6 +1666,8 @@ static void f2fs_apply_options(struct fs_context *fc, struct super_block *sb)
 		F2FS_OPTION(sbi).memory_mode = F2FS_CTX_INFO(ctx).memory_mode;
 	if (ctx->spec_mask & F2FS_SPEC_errors)
 		F2FS_OPTION(sbi).errors = F2FS_CTX_INFO(ctx).errors;
+	if (ctx->spec_mask & F2FS_SPEC_lookup_mode)
+		F2FS_OPTION(sbi).lookup_mode = F2FS_CTX_INFO(ctx).lookup_mode;
 
 	f2fs_apply_compression(fc, sb);
 	f2fs_apply_test_dummy_encryption(fc, sb);
@@ -2416,6 +2432,13 @@ static int f2fs_show_options(struct seq_file *seq, struct dentry *root)
 	if (test_opt(sbi, NAT_BITS))
 		seq_puts(seq, ",nat_bits");
 
+	if (F2FS_OPTION(sbi).lookup_mode == LOOKUP_PERF)
+		seq_show_option(seq, "lookup_mode", "perf");
+	else if (F2FS_OPTION(sbi).lookup_mode == LOOKUP_COMPAT)
+		seq_show_option(seq, "lookup_mode", "compat");
+	else if (F2FS_OPTION(sbi).lookup_mode == LOOKUP_AUTO)
+		seq_show_option(seq, "lookup_mode", "auto");
+
 	return 0;
 }
 
@@ -2480,6 +2503,8 @@ static void default_options(struct f2fs_sb_info *sbi, bool remount)
 #endif
 
 	f2fs_build_fault_attr(sbi, 0, 0, FAULT_ALL);
+
+	F2FS_OPTION(sbi).lookup_mode = LOOKUP_PERF;
 }
 
 #ifdef CONFIG_QUOTA