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Message-ID: <c3ec89a5-5eb5-4358-92eb-27c9406027a2@kernel.org>
Date: Tue, 5 Aug 2025 14:22:12 +0800
From: Chao Yu <chao@...nel.org>
To: Daniel Lee <chullee@...gle.com>, Jaegeuk Kim <jaegeuk@...nel.org>
Cc: chao@...nel.org, linux-f2fs-devel@...ts.sourceforge.net,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] f2fs: add lookup_mode mount option
On 8/5/25 07:05, 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@...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");
Can we show "lookup_mode=perf" in mount option as well?
Thanks,
> + }
> +
> 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
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