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

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Date:   Tue, 7 Dec 2021 07:33:52 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Chengguang Xu <cgxu519@...ernel.net>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        ronyjin <ronyjin@...cent.com>,
        charliecgxu <charliecgxu@...cent.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation

On Sun, Dec 5, 2021 at 4:07 PM Chengguang Xu <cgxu519@...ernel.net> wrote:
>
>  ---- 在 星期四, 2021-12-02 06:47:25 Amir Goldstein <amir73il@...il.com> 撰写 ----
>  > On Wed, Dec 1, 2021 at 6:24 PM Chengguang Xu <cgxu519@...ernel.net> wrote:
>  > >
>  > >  ---- 在 星期三, 2021-12-01 21:46:10 Jan Kara <jack@...e.cz> 撰写 ----
>  > >  > On Wed 01-12-21 09:19:17, Amir Goldstein wrote:
>  > >  > > On Wed, Dec 1, 2021 at 8:31 AM Chengguang Xu <cgxu519@...ernel.net> wrote:
>  > >  > > > So the final solution to handle all the concerns looks like accurately
>  > >  > > > mark overlay inode diry on modification and re-mark dirty only 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 dirty only
>  > >  > > writably mmapped upper AND inode_is_open_for_write(upper)?
>  > >  >
>  > >  > Well, if inode is writeably mapped, it must be also open for write, doesn't
>  > >  > it? The VMA of the mapping will hold file open. So remarking overlay inode
>  > >  > dirty during writeback while inode_is_open_for_write(upper) looks like
>  > >  > reasonably easy and presumably there won't be that many inodes open for
>  > >  > writing for this to become big overhead?
>  >
>  > I think it should be ok and a good tradeoff of complexity vs. performance.
>
> 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 sync should
>  > >  > > be enough. No?
>  > >  >
>  > >  > But we still need to catch other dirtying events like timestamp updates,
>  > >  > truncate(2) etc. to mark overlay inode dirty. Not sure how reliably 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 good.
>
> Currently ovl_copyattr() does not cover all the cases, so I think we still 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.