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

[Jaegeuk Kim]

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@xxxxxxxxxxxxxxxxxxxxx/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?

> 
> 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@xxxxxxxxxx>
> > > ---
> > > 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