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

[Chao Yu <chao@kernel.org>]

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().

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@huawei.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