linux-kernel - RE: [PATCH] exfat: do not clear VolumeDirty in writeback

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Date:   Mon, 28 Feb 2022 10:51:54 +0000
From:   "Yuezhang.Mo@...y.com" <Yuezhang.Mo@...y.com>
To:     "Kohada.Tetsuhiro@...MitsubishiElectric.co.jp" 
        <Kohada.Tetsuhiro@...MitsubishiElectric.co.jp>,
        "linkinjeon@...nel.org" <linkinjeon@...nel.org>,
        "sj1557.seo@...sung.com" <sj1557.seo@...sung.com>
CC:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] exfat: do not clear VolumeDirty in writeback

Hi, Kohada.Tetsuhiro.

Thank for your comments.

>> And VolumeDirty will be set again when updating the parent directory. 
>> It means that BootSector will be written twice in each writeback, that will shorten the life of the device.
> 
> I have the same concern.
> From a lifespan point of view, we should probably clear dirty with just sync_fs().

If it is acceptable for VolumeDirty to remain dirty after all updates are complete, I think it is a good idea.
(PS: The original logic is to clear VolumeDirty after BitMap, FAT and directory entries are updated.)

>>  	sync_blockdev(sb->s_bdev);
>> -	if (exfat_clear_volume_dirty(sb))
>> +	if (__exfat_clear_volume_dirty(sb))
> 
> If SB_SYNCHRONOUS or SB_DIRSYNC is not present, isn't dirty cleared?

With this patch, exfat_clear_volume_dirty() will not clear VolumeDirty if SB_SYNCHRONOUS or SB_DIRSYNC is not present, and __exfat_clear_volume_dirty() will clear VolumeDirty unconditionally.

>> +int exfat_clear_volume_dirty(struct super_block *sb) {
>> +	if (sb->s_flags & (SB_SYNCHRONOUS | SB_DIRSYNC))
>> +		return __exfat_clear_volume_dirty(sb);
> 
> Even when only one of SB or DIR is synced, dirty will be cleared.
> Isn't it necessary to have both SB_SYNCHRONOUS and SB_DIRSYNC?

VolumeDirty will be cleared if one of SB_SYNCHRONOUS and SB_DIRSYNC is set.
The condition of (sb->s_flags & (SB_SYNCHRONOUS | SB_DIRSYNC)) is exactly that.

> And, I think it would be better to use IS_SYNC or IS_DIRSYNC macro here.

If use IS_SYNC or IS_DIRSYNC, we should pass `inode` as an argument, it will be a big change for code.
And if open a file with O_SYNC, IS_DIRSYNC and IS_SYNC will be true, VolumeDirty will be cleared. 
So I think it is not necessary to use IS_DIRSYNC and IS_SYNC.

Best Regards,
Yuezhang,Mo