Re: [f2fs-dev] [PATCH v2] mkfs.f2fs: support -C [no]hashonly to control linear lookup fallback

[Chao Yu <ch...@ke...>]

On 7/25/2025 10:08 AM, Zhiguo Niu wrote:
> Chao Yu via Linux-f2fs-devel <lin...@li...>
> 于2025年7月25日周五 09:03写道：
>>
>> It provides a way to disable linear lookup fallback during mkfs.
>>
>> Behavior summary:
>>                          Android         Distro
>> By default              disabled        enabled
>> Tune w/ [no]hashonly    no              yes
>>
>> Android case:
>>
>> 1.1) Disable linear lookup:
>> mkfs.f2fs -f -g android -O casefold -C utf8:hashonly /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       2 : 2]
>>
>> 1.2) Enable linear lookup:
>> mkfs.f2fs -f -g android -O casefold -C utf8:nohashonly /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       2 : 2]
> Hi Chao,
> Seems like a typo here? should be:

Zhiguo,

Not a typo, it's intentional, see above behavior summary.

But after rethinking about this, I guess it will be good to honor
'nohashonly' for Android case like Distro case.

Anyway, let me update the patch.

Thanks,

> s_encoding_flags                        [0x       0 : 0] ?
> thanks!
>>
>> 1.3) By default:
>> mkfs.f2fs -f -g android -O casefold -C utf8 /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       2 : 2]
>>
>> Distro case:
>>
>> 2.1) Disable linear lookup:
>> mkfs.f2fs -f -O casefold -C utf8:hashonly /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       2 : 2]
>>
>> 2.2) Enable linear lookup:
>> mkfs.f2fs -f -O casefold -C utf8:nohashonly /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       0 : 0]
>>
>> 2.3) By default:
>> mkfs.f2fs -f -O casefold -C utf8 /dev/vdb
>> dump.f2fs -d3 /dev/vdb |grep s_encoding_flags
>> s_encoding_flags                        [0x       0 : 0]
>>
>> Signed-off-by: Chao Yu <ch...@ke...>
>> ---
>> v2:
>> - disable linear lookup by default for Android case
>>   include/f2fs_fs.h       |  3 ++-
>>   lib/libf2fs.c           |  1 +
>>   man/mkfs.f2fs.8         | 10 ++++++++--
>>   mkfs/f2fs_format.c      |  3 +++
>>   mkfs/f2fs_format_main.c |  3 ++-
>>   5 files changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/f2fs_fs.h b/include/f2fs_fs.h
>> index f7268d1..a8da8fa 100644
>> --- a/include/f2fs_fs.h
>> +++ b/include/f2fs_fs.h
>> @@ -1478,7 +1478,8 @@ enum {
>>
>>   /* feature list in Android */
>>   enum {
>> -       F2FS_FEATURE_NAT_BITS = 0x0001,
>> +       F2FS_FEATURE_NAT_BITS           = 0x0001,
>> +       F2FS_FEATURE_LINEAR_LOOKUP      = 0x0002,
>>   };
>>
>>   /* nolinear lookup tune */
>> diff --git a/lib/libf2fs.c b/lib/libf2fs.c
>> index 2f012c8..0e3e62a 100644
>> --- a/lib/libf2fs.c
>> +++ b/lib/libf2fs.c
>> @@ -1424,6 +1424,7 @@ static const struct enc_flags {
>>          char *param;
>>   } encoding_flags[] = {
>>          { F2FS_ENC_STRICT_MODE_FL, "strict" },
>> +       { F2FS_ENC_NO_COMPAT_FALLBACK_FL, "hashonly"}
>>   };
>>
>>   /* Return a positive number < 0xff indicating the encoding magic number
>> diff --git a/man/mkfs.f2fs.8 b/man/mkfs.f2fs.8
>> index 8b3b0cc..8cb7d32 100644
>> --- a/man/mkfs.f2fs.8
>> +++ b/man/mkfs.f2fs.8
>> @@ -232,9 +232,15 @@ Use UTF-8 for casefolding.
>>   .I flags:
>>   .RS 1.2i
>>   .TP 1.2i
>> -.B strict
>> +.B [no]strict
>>   This flag specifies that invalid strings should be rejected by the filesystem.
>> -Default is disabled.
>> +For android case, it will disable linear lookup by default.
>> +.RE
>> +.RS 1.2i
>> +.TP 1.2i
>> +.B [no]hashonly
>> +This flag specifies that there is no linear lookup fallback during lookup.
>> +By default, linear lookup fallback is enabled.
>>   .RE
>>   .RE
>>   .TP
>> diff --git a/mkfs/f2fs_format.c b/mkfs/f2fs_format.c
>> index 2680bd3..a45bbcb 100644
>> --- a/mkfs/f2fs_format.c
>> +++ b/mkfs/f2fs_format.c
>> @@ -671,6 +671,9 @@ static int f2fs_prepare_super_block(void)
>>          memcpy(sb->init_version, c.version, VERSION_LEN);
>>
>>          if (c.feature & F2FS_FEATURE_CASEFOLD) {
>> +               if (!(c.s_encoding_flags & F2FS_ENC_NO_COMPAT_FALLBACK_FL) &&
>> +                       (c.disabled_feature & F2FS_FEATURE_LINEAR_LOOKUP))
>> +                       c.s_encoding_flags |= F2FS_ENC_NO_COMPAT_FALLBACK_FL;
>>                  set_sb(s_encoding, c.s_encoding);
>>                  set_sb(s_encoding_flags, c.s_encoding_flags);
>>          }
>> diff --git a/mkfs/f2fs_format_main.c b/mkfs/f2fs_format_main.c
>> index f0bec4f..8f8e975 100644
>> --- a/mkfs/f2fs_format_main.c
>> +++ b/mkfs/f2fs_format_main.c
>> @@ -143,7 +143,8 @@ static void add_default_options(void)
>>                  force_overwrite = 1;
>>                  c.wanted_sector_size = F2FS_BLKSIZE;
>>                  c.root_uid = c.root_gid = 0;
>> -               c.disabled_feature |= F2FS_FEATURE_NAT_BITS;
>> +               c.disabled_feature |= F2FS_FEATURE_NAT_BITS |
>> +                                       F2FS_FEATURE_LINEAR_LOOKUP;
>>
>>                  /* RO doesn't need any other features */
>>                  if (c.feature & F2FS_FEATURE_RO)
>> --
>> 2.49.0
>>
>>
>>
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