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

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Date:   Thu, 07 Oct 2021 20:26:36 +0800
From:   Chengguang Xu <cgxu519@...ernel.net>
To:     "Jan Kara" <jack@...e.cz>
Cc:     "miklos" <miklos@...redi.hu>, "amir73il" <amir73il@...il.com>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "linux-unionfs" <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 17:01:57 Jan Kara <jack@...e.cz> 撰写 ----
 > On Thu 23-09-21 21:08:10, Chengguang Xu wrote:
 > > Implement overlayfs' ->write_inode to sync dirty data
 > > and redirty overlayfs' inode if necessary.
 > > 
 > > Signed-off-by: Chengguang Xu <cgxu519@...ernel.net>
 > 
 > ...
 > 
 > > +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);
 > > +
 > 
 > I'm somewhat confused here. 'inode' is overlayfs inode AFAIU, so how is it
 > correct to pass it to ->write_inode function of upper filesystem? Shouldn't
 > you pass 'upper' there instead?

That's right!

 > 
 > > +    if (mapping_writably_mapped(upper->i_mapping) ||
 > > +        mapping_tagged(upper->i_mapping, PAGECACHE_TAG_WRITEBACK))
 > > +        iflag |= I_DIRTY_PAGES;
 > > +
 > > +    iflag |= upper->i_state & I_DIRTY_ALL;
 > 
 > Also since you call ->write_inode directly upper->i_state won't be updated
 > to reflect that inode has been written out (I_DIRTY flags get cleared in
 > __writeback_single_inode()). So it seems to me overlayfs will keep writing
 > out upper inode until flush worker on upper filesystem also writes the
 > inode and clears the dirty flags? So you rather need to call something like
 > write_inode_now() that will handle the flag clearing and do writeback list
 > handling for you?
 > 

Calling ->write_inode directly upper->i_state won't be updated, 
however, I don't think overlayfs will keep writing out upper inode since ->write_inode
will be called when only overlay inode itself marked dirty.  Am I missing something?


Thanks,
Chengguang