linux-kernel - Re: [RFC PATCH 13/35] ovl: readd fsync

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Date:   Mon, 23 Apr 2018 09:53:09 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 13/35] ovl: readd fsync

On Mon, Apr 23, 2018 at 03:39:45PM +0200, Miklos Szeredi wrote:
> On Mon, Apr 23, 2018 at 3:36 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > On Thu, Apr 12, 2018 at 05:08:04PM +0200, Miklos Szeredi wrote:
> >> Implement stacked fsync().
> >>
> >> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> >> ---
> >>  fs/overlayfs/file.c | 20 ++++++++++++++++++++
> >>  1 file changed, 20 insertions(+)
> >>
> >> diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> >> index b98204c1c19c..4417527667ff 100644
> >> --- a/fs/overlayfs/file.c
> >> +++ b/fs/overlayfs/file.c
> >> @@ -222,10 +222,30 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> >>       return ret;
> >>  }
> >>
> >> +static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> >> +{
> >> +     struct fd real;
> >> +     const struct cred *old_cred;
> >> +     int ret;
> >> +
> >> +     ret = ovl_real_file(file, &real);
> >> +     if (ret)
> >> +             return ret;
> >> +
> >> +     old_cred = ovl_override_creds(file_inode(file)->i_sb);
> >> +     ret = vfs_fsync_range(real.file, start, end, datasync);
> >> +     revert_creds(old_cred);
> >
> > Can we avoid calling fsync() on real file if it is not upper. Is it worth
> > optimizing.
> 
> Not sure it's worth bothering with.   If caller of fsync(2) didn't
> worry about cost, then why should we?

I was thinking more from the point of view of metadata copy up patches.
For a metacopy file, I was thinking if I can just issue fsync() on upper
file and skip it on lower file.  

Anyway, I don't have any strong opinion here.

Vivek