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Message-ID: <c1472e21-64b1-468e-a98c-ed8e0a5ae8b5@kernel.org>
Date: Mon, 4 Aug 2025 14:32:02 +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 1/2] f2fs: add lookup_mode mount option

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 <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,
> +};

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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