Re: [RFC PATCH] fat: add noflush mount option

[David Timber]

On 8/24/26 18:45, OGAWA Hirofumi wrote:
> David Timber <dxdt@xxxxxxxxxxxx> writes:
>
>> On most distros, udisks manages users' removable volume mount requests.
>> Udisks is configured to always mount FAT volumes with the 'flush' mount
>> option. This is largely okay for populating directory in small sizes,
>> but when a large number of files are involved, the flush behaviour acts
>> as a bottleneck point in fat_file_release(). The user may want to
>> disable the flush option temporarily before commencing such an intensive
>> operation.
>>
>> To cover this use case, introduce the new 'noflush' mount option. When
>> used in the mount options to mount a volume, it overrides the 'flush'
>> option previously specified. When used in remount, update it updates the
>> flag.
>>
>> This is a breaking change as traditionally, the mount options other than
>> rw and ro are ignored. The patch breaks this tradition by allowing
>> reconfiguration of 'flush' and 'noflush' mount options.
>>
>> Signed-off-by: David Timber <dxdt@xxxxxxxxxxxx>
>> ---
>>  fs/fat/inode.c | 26 +++++++++++++++++++++-----
>>  1 file changed, 21 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/fat/inode.c b/fs/fat/inode.c
>> index 28f78df086ef..850df43ba354 100644
>> --- a/fs/fat/inode.c
>> +++ b/fs/fat/inode.c
>> @@ -813,10 +813,14 @@ int fat_reconfigure(struct fs_context *fc)
>>  	bool new_rdonly;
>>  	struct super_block *sb = fc->root->d_sb;
>>  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
>> +	struct fat_mount_options *new_opts = fc->fs_private;
>>  	fc->sb_flags |= SB_NODIRATIME | (sbi->options.isvfat ? 0 : SB_NOATIME);
>>  
>>  	sync_filesystem(sb);
>>  
>> +	/* allow reconfiguring "flush" or "noflush" */
>> +	sbi->options.flush = new_opts->flush;
>> +
>>  	/* make sure we update state on remount. */
>>  	new_rdonly = fc->sb_flags & SB_RDONLY;
>>  	if (new_rdonly != sb_rdonly(sb)) {
> Maybe, better to set after changed the read-only?
ACK

>> @@ -1047,7 +1051,7 @@ enum {
>>  	Opt_charset, Opt_shortname, Opt_utf8, Opt_utf8_bool,
>>  	Opt_uni_xl, Opt_uni_xl_bool, Opt_nonumtail, Opt_nonumtail_bool,
>>  	Opt_obsolete, Opt_flush, Opt_tz, Opt_rodir, Opt_errors, Opt_discard,
>> -	Opt_nfs, Opt_nfs_enum, Opt_time_offset, Opt_dos1xfloppy,
>> +	Opt_nfs, Opt_nfs_enum, Opt_time_offset, Opt_dos1xfloppy, Opt_noflush
>>  };
>>  
>>  static const struct constant_table fat_param_check[] = {
>> @@ -1110,6 +1114,7 @@ const struct fs_parameter_spec fat_param_spec[] = {
>>  	fsparam_flag	("debug",	Opt_debug),
>>  	fsparam_flag	("sys_immutable", Opt_immutable),
>>  	fsparam_flag	("flush",	Opt_flush),
>> +	fsparam_flag	("noflush",	Opt_noflush),
> fsparam_flag() is not including the "noflush" too?
I think I should use fsparam_flag_no() here. My bad.

It seems that you don't object to the idea. That's a good start!

If we want to do this, the
documentation(Documentation/filesystems/vfat.rst and mount(8)) should be
updated, too. fyi, `udisksctl mount -o` won't accept "noflush" so
that'll have to be fixed, too. Also have to make sure if "noflush" is
used in fstab, udisks2 overrides the system default settings.

Davo