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

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Message-ID: <20211008131351.GA15930@quack2.suse.cz>
Date:   Fri, 8 Oct 2021 15:13:51 +0200
From:   Jan Kara <jack@...e.cz>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Chengguang Xu <cgxu519@...ernel.net>, 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 07-10-21 20:51:47, Miklos Szeredi wrote:
> On Thu, 7 Oct 2021 at 16:53, Chengguang Xu <cgxu519@...ernel.net> wrote:
> >
> >  ---- 在 星期四, 2021-10-07 22:46:46 Jan Kara <jack@...e.cz> 撰写 ----
> >  > On Thu 07-10-21 15:34:19, Miklos Szeredi wrote:
> >  > > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@...ernel.net> wrote:
> >  > > >  > 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).
> >  > >
> >  > > Right.
> >  > >
> >  > > But wouldn't it be simpler to do this from ->write_inode()?
> >  >
> >  > You could but then you'd have to make sure you have I_DIRTY_SYNC always set
> >  > when I_DIRTY_PAGES is set on the upper inode so that your ->write_inode()
> >  > callback gets called. Overall I agree the logic would be probably simpler.
> >  >
> >
> 
> And it's not just for simplicity.  The I_SYNC logic in
> writeback_single_inode() is actually necessary to prevent races
> between instances on a specific inode.  I.e. if inode writeback is
> started by background wb then syncfs needs to synchronize with that
> otherwise it will miss the inode, or worse, mess things up by calling
> ->write_inode() multiple times in parallel.  So going throught
> writeback_single_inode() is actually a must AFAICS.

Yes, you are correct.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR