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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 16 Nov 2021 13:35:55 +0100
Message-ID: <CAJfpegttQreuuD_jLgJmrYpsLKBBe2LmB5NSj6F5dHoTzqPArw@mail.gmail.com>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation
To:     Chengguang Xu <cgxu519@mykernel.net>
Cc:     Jan Kara <jack@suse.cz>, Amir Goldstein <amir73il@gmail.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, 16 Nov 2021 at 03:20, Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E5=9B=9B, 2021-10-07 21:34:19 Miklos S=
zeredi <miklos@szeredi.hu> =E6=92=B0=E5=86=99 ----
>  > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@mykernel.net> wrot=
e:
>  > >  > However that wasn't what I was asking about.  AFAICS ->write_inod=
e()
>  > >  > won't start write back for dirty pages.   Maybe I'm missing somet=
hing,
>  > >  > 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 overlay=
fs ->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=EF=BC=8C
>
> 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 ov=
erlay 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