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 <chul...@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
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