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In-Reply-To: <17d8aeb19ac.f22523af26365.6531629287230366441@mykernel.net>
From:   Amir Goldstein <amir73il@gmail.com>
Date:   Tue, 7 Dec 2021 07:33:52 +0200
Message-ID: <CAOQ4uxgwZoB5GQJZvpPLzRqrQA-+JSowD+brUwMSYWf9zZjiRQ@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>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        ronyjin <ronyjin@tencent.com>,
        charliecgxu <charliecgxu@tencent.com>,
        Vivek Goyal <vgoyal@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>
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On Sun, Dec 5, 2021 at 4:07 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>
>  ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E5=9B=9B, 2021-12-02 06:47:25 Amir Gol=
dstein <amir73il@gmail.com> =E6=92=B0=E5=86=99 ----
>  > On Wed, Dec 1, 2021 at 6:24 PM Chengguang Xu <cgxu519@mykernel.net> wr=
ote:
>  > >
>  > >  ---- =E5=9C=A8 =E6=98=9F=E6=9C=9F=E4=B8=89, 2021-12-01 21:46:10 Jan=
 Kara <jack@suse.cz> =E6=92=B0=E5=86=99 ----
>  > >  > On Wed 01-12-21 09:19:17, Amir Goldstein wrote:
>  > >  > > On Wed, Dec 1, 2021 at 8:31 AM Chengguang Xu <cgxu519@mykernel.=
net> wrote:
>  > >  > > > So the final solution to handle all the concerns looks like a=
ccurately
>  > >  > > > mark overlay inode diry on modification and re-mark dirty onl=
y for
>  > >  > > > mmaped file in ->write_inode().
>  > >  > > >
>  > >  > > > Hi Miklos, Jan
>  > >  > > >
>  > >  > > > Will you agree with new proposal above?
>  > >  > > >
>  > >  > >
>  > >  > > Maybe you can still pull off a simpler version by remarking dir=
ty only
>  > >  > > writably mmapped upper AND inode_is_open_for_write(upper)?
>  > >  >
>  > >  > Well, if inode is writeably mapped, it must be also open for writ=
e, doesn't
>  > >  > it? The VMA of the mapping will hold file open. So remarking over=
lay inode
>  > >  > dirty during writeback while inode_is_open_for_write(upper) looks=
 like
>  > >  > reasonably easy and presumably there won't be that many inodes op=
en for
>  > >  > writing for this to become big overhead?
>  >
>  > I think it should be ok and a good tradeoff of complexity vs. performa=
nce.
>
> IMO, mark dirtiness on write is relatively simple, so I think we can mark=
 the
> overlayfs inode dirty during real write behavior and only remark writable=
 mmap
> unconditionally in ->write_inode().
>

If by "on write" you mean on write/copy_file_range/splice_write/...
then yes I agree
since we have to cover all other mnt_want_write() cases anyway.

>
>  >
>  > >  >
>  > >  > > If I am not mistaken, if you always mark overlay inode dirty on=
 ovl_flush()
>  > >  > > of FMODE_WRITE file, there is nothing that can make upper inode=
 dirty
>  > >  > > after last close (if upper is not mmaped), so one more inode sy=
nc should
>  > >  > > be enough. No?
>  > >  >
>  > >  > But we still need to catch other dirtying events like timestamp u=
pdates,
>  > >  > truncate(2) etc. to mark overlay inode dirty. Not sure how reliab=
ly that
>  > >  > can be done...
>  > >  >
>  >
>  > Oh yeh, we have those as well :)
>  > All those cases should be covered by ovl_copyattr() that updates the
>  > ovl inode ctime/mtime, so always dirty in ovl_copyattr() should be goo=
d.
>
> Currently ovl_copyattr() does not cover all the cases, so I think we stil=
l need to carefully
> check all the places of calling mnt_want_write().
>

Careful audit is always good, but if we do not have ovl_copyattr() in
a call site
that should mark inode dirty, then it sounds like a bug, because ovl inode =
ctime
will not get updated. Do you know of any such cases?

Thanks,
Amir.