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

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Message-ID: <17d76821691.c8e249b322113.2136207110726046721@mykernel.net>
Date:   Wed, 01 Dec 2021 22:59:36 +0800
From:   Chengguang Xu <cgxu519@...ernel.net>
To:     "Jan Kara" <jack@...e.cz>
Cc:     "Amir Goldstein" <amir73il@...il.com>,
        "Miklos Szeredi" <miklos@...redi.hu>,
        "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>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode
 operation

 ---- 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? 

That's right.


 > The VMA of the mapping will hold file open. 

It's a bit tricky but currently ovl_mmap() will replace file to realfile in upper layer
and release overlayfs file. So overlayfs file itself will not have any relationship with
the VMA anymore after mmap().


Thanks,
Chengguang


 > 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?
 > 
 > > 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...
 > 
 >                                 Honza
 > -- 
 > Jan Kara <jack@...e.com>
 > SUSE Labs, CR
 >