linux-kernel - Re: [PATCH] exfat: remove EXFAT_SB

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Message-ID: <b29d254b-212a-bfcb-ab7c-456f481b85c8@gmail.com>
Date:   Fri, 12 Jun 2020 19:22:34 +0900
From:   Tetsuhiro Kohada <kohada.t2@...il.com>
To:     Sungjong Seo <sj1557.seo@...sung.com>
Cc:     kohada.tetsuhiro@...mitsubishielectric.co.jp,
        mori.takahiro@...mitsubishielectric.co.jp,
        motai.hirotaka@...mitsubishielectric.co.jp,
        'Namjae Jeon' <namjae.jeon@...sung.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exfat: remove EXFAT_SB_DIRTY flag

On 2020/06/12 17:34, Sungjong Seo wrote:
>> remove EXFAT_SB_DIRTY flag and related codes.
>>
>> This flag is set/reset in exfat_put_super()/exfat_sync_fs() to avoid
>> sync_blockdev().
>> However ...
>> - exfat_put_super():
>> Before calling this, the VFS has already called sync_filesystem(), so sync
>> is never performed here.
>> - exfat_sync_fs():
>> After calling this, the VFS calls sync_blockdev(), so, it is meaningless
>> to check EXFAT_SB_DIRTY or to bypass sync_blockdev() here.
>> Not only that, but in some cases can't clear VOL_DIRTY.
>> ex:
>> VOL_DIRTY is set when rmdir starts, but when non-empty-dir is detected,
>> return error without setting EXFAT_SB_DIRTY.
>> If performe 'sync' in this state, VOL_DIRTY will not be cleared.
>>
>> Remove the EXFAT_SB_DIRTY check to ensure synchronization.
>> And, remove the code related to the flag.
>>
>> Signed-off-by: Tetsuhiro Kohada <kohada.t2@...il.com>
>> ---
>>   fs/exfat/balloc.c   |  4 ++--
>>   fs/exfat/dir.c      | 16 ++++++++--------
>>   fs/exfat/exfat_fs.h |  5 +----
>>   fs/exfat/fatent.c   |  7 ++-----
>>   fs/exfat/misc.c     |  3 +--
>>   fs/exfat/namei.c    | 12 ++++++------
>>   fs/exfat/super.c    | 11 +++--------
>>   7 files changed, 23 insertions(+), 35 deletions(-)
>>
> [snip]
>>
>> @@ -62,11 +59,9 @@ static int exfat_sync_fs(struct super_block *sb, int
>> wait)
>>
>>   	/* If there are some dirty buffers in the bdev inode */
>>   	mutex_lock(&sbi->s_lock);
>> -	if (test_and_clear_bit(EXFAT_SB_DIRTY, &sbi->s_state)) {
>> -		sync_blockdev(sb->s_bdev);
>> -		if (exfat_set_vol_flags(sb, VOL_CLEAN))
>> -			err = -EIO;
>> -	}
> 
> I looked through most codes related to EXFAT_SB_DIRTY and VOL_DIRTY.
> And your approach looks good because all of them seem to be protected by
> s_lock.
> 
> BTW, as you know, sync_filesystem() calls sync_fs() with 'nowait' first,
> and then calls it again with 'wait' twice. No need to sync with lock twice.
> If so, isn't it okay to do nothing when wait is 0?

I also think  ‘do nothing when wait is 0’ as you say, but I'm still not sure.

Some other Filesystems do nothing with nowait and just return.
However, a few Filesystems always perform sync.

sync_blockdev() waits for completion, so it may be inappropriate to call with  nowait. (But it was called in the original code)

I'm still not sure, so I excluded it in this patch.
Is it okay to include it?


>> +	sync_blockdev(sb->s_bdev);
>> +	if (exfat_set_vol_flags(sb, VOL_CLEAN))
>> +		err = -EIO;
>>   	mutex_unlock(&sbi->s_lock);
>>   	return err;
>>   }
>> --
>> 2.25.1
> 
> 

BR
---
Tetsuhiro Kohada <kohada.t2@...il.com>