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

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Date:   Fri, 2 Jul 2021 23:49:25 +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 2021/7/2 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/

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