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Date:   Tue, 31 Mar 2020 10:34:30 +0200
From:   Karel Zak <kzak@redhat.com>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, dray@redhat.com,
        Miklos Szeredi <mszeredi@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Jeff Layton <jlayton@redhat.com>, Ian Kent <raven@themaw.net>,
        andres@anarazel.de, keyrings@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lennart Poettering <lennart@poettering.net>,
        Aleksa Sarai <cyphar@cyphar.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
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On Tue, Mar 31, 2020 at 07:11:11AM +0200, Miklos Szeredi wrote:
> On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner
> <christian.brauner@ubuntu.com> wrote:
> 
> > Fwiw, putting down my kernel hat and speaking as someone who maintains
> > two container runtimes and various other low-level bits and pieces in
> > userspace who'd make heavy use of this stuff I would prefer the fd-based
> > fsinfo() approach especially in the light of across namespace
> > operations, querying all properties of a mount atomically all-at-once,
> 
> fsinfo(2) doesn't meet the atomically all-at-once requirement.

I guess your /proc based idea have exactly the same problem... 

I see two possible ways:

- after open("/mnt", O_PATH) create copy-on-write object in kernel to
  represent mount node -- kernel will able to modify it, but userspace
  will get unchanged data from the FD until to close()

- improve fsinfo() to provide set (list) of the attributes by one call

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com