linux-kernel - Re: [PATCH v2 RFC] f2fs: fix to force keeping write barrier for strict fsync mode

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Date:   Tue, 13 Jul 2021 17:23:32 +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 2021/7/8 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

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