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

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Date:   Thu, 07 Oct 2021 21:09:55 +0800
From:   Chengguang Xu <cgxu519@...ernel.net>
To:     "Miklos Szeredi" <miklos@...redi.hu>
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

 ---- 在 星期四, 2021-10-07 20:45:20 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
 > 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.

It's a little bit weird this passed all overlay cases and generic/474(syncfs) without errors in xfstests.
Please let me do more diagnosis on this and strengthen the test case.


 > 
 > 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.
 > 

Actually, page writeback on upper inode will be triggered by overlayfs ->writepages and
overlayfs' ->writepages will be called by vfs writeback function (i.e writeback_sb_inodes).

Thanks,
Chengguang