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Message-ID: <20250804230558.746338-1-chullee@google.com>
Date: Mon, 4 Aug 2025 16:05:57 -0700
From: Daniel Lee <chullee@...gle.com>
To: Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>
Cc: linux-f2fs-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
Daniel Lee <chullee@...gle.com>
Subject: [PATCH v2 1/2] 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@...gle.com>
---
v2:
- rework mount option parsing to use the new mount API.
- add lookup_mode field to struct f2fs_mount_info.
- add show_options support for the new option.
Documentation/filesystems/f2fs.rst | 19 +++++++++++++++++++
fs/f2fs/dir.c | 17 ++++++++++++++++-
fs/f2fs/f2fs.h | 7 +++++++
fs/f2fs/super.c | 25 +++++++++++++++++++++++++
4 files changed, 67 insertions(+), 1 deletion(-)
diff --git a/Documentation/filesystems/f2fs.rst b/Documentation/filesystems/f2fs.rst
index 03b1efa6d3b2..a80ed82a547a 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 fffd7749d6d1..48f4f98afb01 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_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 @@ 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 7029aa8b430e..1c0edb8a7134 100644
--- a/fs/f2fs/f2fs.h
+++ b/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
@@ -1448,6 +1449,12 @@ 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);
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index e16c4e2830c2..ed17ab4d5cef 100644
--- a/fs/f2fs/super.c
+++ b/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) {
+ 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
--
2.50.1.565.gc32cd1483b-goog
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