Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation

From: Chengguang Xu <cgxu519@mykernel.net>
To: Amir Goldstein <amir73il@gmail.com>
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>
Subject: Re: [RFC PATCH v5 06/10] ovl: implement overlayfs' ->write_inode operation
Date: Sun, 6 Feb 2022 00:09:39 +0800	[thread overview]
Message-ID: <362c02fa-2625-30c4-17a1-1a95753b6065@mykernel.net> (raw)
In-Reply-To: <CAOQ4uxgwZoB5GQJZvpPLzRqrQA-+JSowD+brUwMSYWf9zZjiRQ@mail.gmail.com>

在 2021/12/7 13:33, Amir Goldstein 写道:
> On Sun, Dec 5, 2021 at 4:07 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>>   ---- 在 星期四, 2021-12-02 06:47:25 Amir Goldstein <amir73il@gmail.com> 撰写 ----
>>   > On Wed, Dec 1, 2021 at 6:24 PM Chengguang Xu <cgxu519@mykernel.net> wrote:
>>   > >
>>   > >  ---- 在 星期三, 2021-12-01 21:46:10 Jan Kara <jack@suse.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@mykernel.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?

Sorry for my late response, I've been very busy lately.
For your question, for example, there is a case of calling 
ovl_want_write() in ovl_cache_get_impure() and caller does not call 
ovl_copyattr()
so I think we should explicitly mark ovl inode dirty in that case. Is 
that probably a bug?

Thanks,
Chengguang