linux-kernel - Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write

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Date:   Thu, 7 Oct 2021 14:45:20 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation

On Thu, 7 Oct 2021 at 14:28, Chengguang Xu <cgxu519@...ernel.net> wrote:
>
>  ---- 在 星期四, 2021-10-07 17:23:06 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
>  > On Thu, 23 Sept 2021 at 15:08, Chengguang Xu <cgxu519@...ernel.net> wrote:
>  > >
>  > > Implement overlayfs' ->write_inode to sync dirty data
>  > > and redirty overlayfs' inode if necessary.
>  > >
>  > > Signed-off-by: Chengguang Xu <cgxu519@...ernel.net>
>  > > ---
>  > >  fs/overlayfs/super.c | 30 ++++++++++++++++++++++++++++++
>  > >  1 file changed, 30 insertions(+)
>  > >
>  > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
>  > > index 2ab77adf7256..cddae3ca2fa5 100644
>  > > --- a/fs/overlayfs/super.c
>  > > +++ b/fs/overlayfs/super.c
>  > > @@ -412,12 +412,42 @@ static void ovl_evict_inode(struct inode *inode)
>  > >         clear_inode(inode);
>  > >  }
>  > >
>  > > +static int ovl_write_inode(struct inode *inode,
>  > > +                          struct writeback_control *wbc)
>  > > +{
>  > > +       struct ovl_fs *ofs = inode->i_sb->s_fs_info;
>  > > +       struct inode *upper = ovl_inode_upper(inode);
>  > > +       unsigned long iflag = 0;
>  > > +       int ret = 0;
>  > > +
>  > > +       if (!upper)
>  > > +               return 0;
>  > > +
>  > > +       if (!ovl_should_sync(ofs))
>  > > +               return 0;
>  > > +
>  > > +       if (upper->i_sb->s_op->write_inode)
>  > > +               ret = upper->i_sb->s_op->write_inode(inode, wbc);
>  >
>  > Where is page writeback on upper inode triggered?
>  >
>
> Should pass upper inode instead of overlay inode here.

That's true and it does seem to indicate lack of thorough testing.

However that wasn't what I was asking about.  AFAICS ->write_inode()
won't start write back for dirty pages.   Maybe I'm missing something,
but there it looks as if nothing will actually trigger writeback for
dirty pages in upper inode.

Thanks,
Miklos