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

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Message-ID: <CAJfpegvek6=+Xk+jLNYnH0piQKRqb9CWst_aNHWExZeq+7jOQw@mail.gmail.com>
Date:   Thu, 7 Oct 2021 20:51:47 +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 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.

Thanks,
Miklos