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

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Message-ID: <17c5b3fa20a.ff02131b26074.9058176981832458952@mykernel.net>
Date:   Thu, 07 Oct 2021 22:54:29 +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 22:41:56 Jan Kara <jack@...e.cz> 撰写 ----
 > On Thu 07-10-21 20:26:36, Chengguang Xu wrote:
 > >  ---- 在 星期四, 2021-10-07 17:01:57 Jan Kara <jack@...e.cz> 撰写 ----
 > >  > 
 > >  > > +    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?
 > 
 > Well, if upper->i_state is not updated, you are more or less guaranteed
 > upper->i_state & I_DIRTY_ALL != 0 and thus even overlay inode stays dirty.
 > And thus next time writeback runs you will see dirty overlay inode and
 > writeback the upper inode again although it is not necessary.
 > 

Hi Jan,

Yes, I get the point now. Thanks for the explanation.


Thanks,
Chengguang