Date:   Wed, 17 Nov 2021 14:11:29 +0800
From:   Chengguang Xu <cgxu519@mykernel.net>
Reply-To: cgxu519@mykernel.net
To:     "Miklos Szeredi" <miklos@szeredi.hu>
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>
Message-ID: <17d2c858d76.d8a27d876510.8802992623030721788@mykernel.net>
In-Reply-To: <CAJfpegttQreuuD_jLgJmrYpsLKBBe2LmB5NSj6F5dHoTzqPArw@mail.gmail.com>
References: <20210923130814.140814-1-cgxu519@mykernel.net> <20210923130814.140814-7-cgxu519@mykernel.net>
 <CAJfpeguqj2vst4Zj5EovSktJkXiDSCSWY=X12X0Yrz4M8gPRmQ@mail.gmail.com>
 <17c5aba1fef.c5c03d5825886.6577730832510234905@mykernel.net>
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 <17c5adfe5ea.12f1be94625921.4478415437452327206@mykernel.net>
 <CAJfpegt4jZpSCXGFk2ieqUXVm3m=ng7QtSzZp2bXVs07bfrbXg@mail.gmail.com> <17d268ba3ce.1199800543649.1713755891767595962@mykernel.net> <CAJfpegttQreuuD_jLgJmrYpsLKBBe2LmB5NSj6F5dHoTzqPArw@mail.gmail.com>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
 operation
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 ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E4=BA=8C, 2021-11-16 20:35:55 Miklos Sze=
redi <miklos@szeredi.hu> =E6=92=B0=E5=86=99 ----
 > 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 Miklo=
s Szeredi <miklos@szeredi.hu> =E6=92=B0=E5=86=99 ----
 > >  > On Thu, 7 Oct 2021 at 15:10, Chengguang Xu <cgxu519@mykernel.net> w=
rote:
 > >  > >  > However that wasn't what I was asking about.  AFAICS ->write_i=
node()
 > >  > >  > won't start write back for dirty pages.   Maybe I'm missing so=
mething,
 > >  > >  > but there it looks as if nothing will actually trigger writeba=
ck for
 > >  > >  > dirty pages in upper inode.
 > >  > >  >
 > >  > >
 > >  > > Actually, page writeback on upper inode will be triggered by over=
layfs ->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 proje=
ct.
 > >
 > > 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 sim=
ple enough.
 >=20
 > Are you suggesting that all non-lower overlay inodes should always be di=
rty?
 >=20
 > 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? =20

That's true.

 > 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.
 >=20

No problem, I'll do some tests for the performance.

Thanks,
Chengguang