Date:   Wed, 14 Jul 2021 10:51:18 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 RFC] f2fs: fix to force keeping write barrier for
 strict fsync mode

On 2021/7/14 10:19, Jaegeuk Kim wrote:
> On 07/14, Chao Yu wrote:
>> On 2021/7/14 7:34, Jaegeuk Kim wrote:
>>> On 07/13, Chao Yu wrote:
>>>> On 2021/7/8 1:48, Jaegeuk Kim wrote:
>>>>> On 07/02, Chao Yu wrote:
>>>>>> On 2021/7/2 9:32, Jaegeuk Kim wrote:
>>>>>>> On 07/02, Chao Yu wrote:
>>>>>>>> On 2021/7/2 1:10, Jaegeuk Kim wrote:
>>>>>>>>> On 06/01, Chao Yu wrote:
>>>>>>>>>> [1] https://www.mail-archive.com/linux-f2fs-devel@lists.sourceforge.net/msg15126.html
>>>>>>>>>>
>>>>>>>>>> As [1] reported, if lower device doesn't support write barrier, in below
>>>>>>>>>> case:
>>>>>>>>>>
>>>>>>>>>> - write page #0; persist
>>>>>>>>>> - overwrite page #0
>>>>>>>>>> - fsync
>>>>>>>>>>       - write data page #0 OPU into device's cache
>>>>>>>>>>       - write inode page into device's cache
>>>>>>>>>>       - issue flush
>>>>>>>>>
>>>>>>>>> Well, we have preflush for node writes, so I don't think this is the case.
>>>>>>>>>
>>>>>>>>>       fio.op_flags |= REQ_PREFLUSH | REQ_FUA;
>>>>>>>>
>>>>>>>> This is only used for atomic write case, right?
>>>>>>>>
>>>>>>>> I mean the common case which is called from f2fs_issue_flush() in
>>>>>>>> f2fs_do_sync_file().
>>>>>>>
>>>>>>> How about adding PREFLUSH when writing node blocks aligned to the above set?
>>>>>>
>>>>>> You mean implementation like v1 as below?
>>>>>>
>>>>>> https://lore.kernel.org/linux-f2fs-devel/20200120100045.70210-1-yuchao0@huawei.com/
>>>>>
>>>>> Yea, I think so. :P
>>>>
>>>> I prefer v2, we may have several schemes to improve performance with v2, e.g.
>>>> - use inplace IO to avoid newly added preflush
>>>> - use flush_merge option to avoid redundant preflush
>>>> - if lower device supports barrier IO, we can avoid newly added preflush
>>>
>>> Doesn't v2 give one more flush than v1? Why do you want to take worse one and
>>
>> FUA implies an extra preflush command or similar mechanism in lower device to keep data
>> in bio being persistent before this command's completion.
>>
>> Also if lower device doesn't support FUA natively, block layer turns it into an empty
>> PREFLUSH command.
>>
>> So, it's hard to say which one will win the benchmark game, maybe we need some
>> performance data before making the choice, but you know, it depends on device's
>> character.
> 
> I was looking at # of bios.
> 
>>
>>> try to improve back? Not clear the benefit on v2.
>>
>> Well, if user suffer and complain performance regression with v1, any plan to improve it?
>>
>> I just thought about plan B/C/D for no matter v1 or v2.
> 
> I assumed you wanted v2 since it might be used for B/C/D improvements. But, it
> seems it wasn't. My point is to save one bio, but piggyback the flag to the
> device driver.

I doubt the conclusion...but it needs to get some data to prove it.

I think the right way is merging v1 now to fix the bug firstly, and let me do
the comparison on them a little bit later to see whether we need another
implementation... thoughts?

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> And please see do_checkpoint(), we call f2fs_flush_device_cache() and
>>>>>>>> commit_checkpoint() separately to keep persistence order of CP datas.
>>>>>>>>
>>>>>>>> See commit 46706d5917f4 ("f2fs: flush cp pack except cp pack 2 page at first")
>>>>>>>> for details.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> If SPO is triggered during flush command, inode page can be persisted
>>>>>>>>>> before data page #0, so that after recovery, inode page can be recovered
>>>>>>>>>> with new physical block address of data page #0, however there may
>>>>>>>>>> contains dummy data in new physical block address.
>>>>>>>>>>
>>>>>>>>>> Then what user will see is: after overwrite & fsync + SPO, old data in
>>>>>>>>>> file was corrupted, if any user do care about such case, we can suggest
>>>>>>>>>> user to use STRICT fsync mode, in this mode, we will force to trigger
>>>>>>>>>> preflush command to persist data in device cache in prior to node
>>>>>>>>>> writeback, it avoids potential data corruption during fsync().
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Chao Yu <yuchao0@...wei.com>
>>>>>>>>>> ---
>>>>>>>>>> v2:
>>>>>>>>>> - fix this by adding additional preflush command rather than using
>>>>>>>>>> atomic write flow.
>>>>>>>>>>       fs/f2fs/file.c | 14 ++++++++++++++
>>>>>>>>>>       1 file changed, 14 insertions(+)
>>>>>>>>>>
>>>>>>>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>>>>>>>> index 7d5311d54f63..238ca2a733ac 100644
>>>>>>>>>> --- a/fs/f2fs/file.c
>>>>>>>>>> +++ b/fs/f2fs/file.c
>>>>>>>>>> @@ -301,6 +301,20 @@ static int f2fs_do_sync_file(struct file *file, loff_t start, loff_t end,
>>>>>>>>>>       				f2fs_exist_written_data(sbi, ino, UPDATE_INO))
>>>>>>>>>>       			goto flush_out;
>>>>>>>>>>       		goto out;
>>>>>>>>>> +	} else {
>>>>>>>>>> +		/*
>>>>>>>>>> +		 * for OPU case, during fsync(), node can be persisted before
>>>>>>>>>> +		 * data when lower device doesn't support write barrier, result
>>>>>>>>>> +		 * in data corruption after SPO.
>>>>>>>>>> +		 * So for strict fsync mode, force to trigger preflush to keep
>>>>>>>>>> +		 * data/node write order to avoid potential data corruption.
>>>>>>>>>> +		 */
>>>>>>>>>> +		if (F2FS_OPTION(sbi).fsync_mode == FSYNC_MODE_STRICT &&
>>>>>>>>>> +								!atomic) {
>>>>>>>>>> +			ret = f2fs_issue_flush(sbi, inode->i_ino);
>>>>>>>>>> +			if (ret)
>>>>>>>>>> +				goto out;
>>>>>>>>>> +		}
>>>>>>>>>>       	}
>>>>>>>>>>       go_write:
>>>>>>>>>>       	/*
>>>>>>>>>> -- 
>>>>>>>>>> 2.29.2

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