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

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Message-ID: <CAJfpegttQreuuD_jLgJmrYpsLKBBe2LmB5NSj6F5dHoTzqPArw@mail.gmail.com>
Date:   Tue, 16 Nov 2021 13:35:55 +0100
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 Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@...ernel.net> wrote:
>
>  ---- 在 星期四, 2021-10-07 21:34:19 Miklos Szeredi <miklos@...redi.hu> 撰写 ----
>  > 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()?
>  >
>  > I.e. call write_inode_now() as suggested by Jan.
>  >
>  > Also could just call mark_inode_dirty() on the overlay inode
>  > regardless of the dirty flags on the upper inode since it shouldn't
>  > matter and results in simpler logic.
>  >
>
> Hi Miklos，
>
> Sorry for delayed response for this, I've been busy with another project.
>
> I agree with your suggesion above and further more how about just mark overlay inode dirty
> when it has upper inode? This approach will make marking dirtiness simple enough.

Are you suggesting that all non-lower overlay inodes should always be dirty?

The logic would be simple, no doubt, but there's the cost to walking
those overlay inodes which don't have a dirty upper inode, right?  Can
you quantify this cost with a benchmark?  Can be totally synthetic,
e.g. lookup a million upper files without modifying them, then call
syncfs.

Thanks,
Miklos