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Subject: Re: [f2fs-dev] [PATCH] f2fs: don't clear CP_QUOTA_NEED_FSCK_FLAG
To:     Jaegeuk Kim <jaegeuk@kernel.org>
CC:     <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>
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 <29a4fd24-6340-41df-7199-6e89b70c6341@huawei.com>
 <20190220070855.GA91331@jaegeuk-macbookpro.roam.corp.google.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <c39a995f-9bfd-e68a-7d04-11bd8c0f8e64@huawei.com>
Date:   Wed, 20 Feb 2019 15:25:08 +0800
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On 2019/2/20 15:08, Jaegeuk Kim wrote:
> On 02/18, Chao Yu wrote:
>> On 2019/2/16 12:55, Jaegeuk Kim wrote:
>>> On 02/13, Chao Yu wrote:
>>>> On 2019/2/12 10:33, Jaegeuk Kim wrote:
>>>>> If we met this once, let fsck.f2fs clear this only.
>>>>> Note that, this addresses all the subtle fault injection test.
>>>>>
>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>>>> ---
>>>>>  fs/f2fs/checkpoint.c | 2 --
>>>>>  1 file changed, 2 deletions(-)
>>>>>
>>>>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>>>>> index 03fea4efd64b..10a3ada28715 100644
>>>>> --- a/fs/f2fs/checkpoint.c
>>>>> +++ b/fs/f2fs/checkpoint.c
>>>>> @@ -1267,8 +1267,6 @@ static void update_ckpt_flags(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>>>>>  
>>>>>  	if (is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH))
>>>>>  		__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>>>>> -	else
>>>>> -		__clear_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>>>>
>>>> I didn't get it, previously, if we didn't persist all quota file's data in
>>>> checkpoint, then we will tag CP_QUOTA_NEED_FSCK_FLAG in CP area, but in current
>>>> checkpoint, we have persisted all quota file's data, quota files are consistent
>>>> with all other files in filesystem, why we can't remove this NEED_FSCK flag..?
>>>
>>> I said it's subtle. So, I guessed 1) set CP_QUOTA_NEED_FSCK_FLAG, 2) clear
>>
>> I know it's subtle... and I agreed we can fix it like this in upstream
>> first, but in our product, it's not rare that we hit the
>> QUOTA_SKIP_FLUSH(its value is 4) case, later we may encounter long latency
>> caused by quota repairing of fsck.
>>
>>> SBI_QUOTA_SKIP_FLUSH by checkpoint, 3) clear CP_QUOTA_NEED_FSCK_FLAG by another
>>> checkpoint?
>>
>> But later if QUOTA_NEED_REPAIR is set, we will set QUOTA_NEED_FSCK_FLAG
>> again, right?
>>
>> if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR))
>> 	__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>>
>>
>> So in order to figure out whether this is caused by out-of-order execution
>> of below assignments:
>>
>> 	if (is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH))
>> 		__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>> 	else
>> 		__clear_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG); --- clear flag later
>>
>> 	if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR))
>> 		__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG); --- set flag first
>>
>>
>> Could you have a try:
>>
>> 	if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR) ||
>> 			is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH))
>> 		__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>> 	else
>> 		__clear_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
> 
> What does this mean? I'm in doubt we have a missing case where we clear this

Cpu pipeline / compiler can make code out-of-order execution, which means:

a = 1;
b = 2;

may actually be executed as the order of:

b = 2;
a = 1;

So I doubt that below two line codes can be executed out-of-order:

else
	__clear_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG); --- clear flag later

if ()
	__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG); --- set flag first

> flag, CP_QUOTA_NEED_FSCK_FLAG.

Agreed, I've checked each operation in f2fs_quota_operations yesterday, and
didn't find any missing places yet...

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>  
>>>>>  	if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR))
>>>>>  		__set_ckpt_flags(ckpt, CP_QUOTA_NEED_FSCK_FLAG);
>>>>>
>>>
>>> .
>>>
> 
> .
>