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Message-ID: <20250804060327.512247-1-chullee@google.com>
Date: Sun, 3 Aug 2025 23:03:26 -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 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>
---
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
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