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

[Chao Yu]

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().

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