linux-kernel - Re: Upcoming: Notifications, FS notifications and fsinfo()

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Message-ID: <20200331083430.kserp35qabnxvths@ws.net.home>
Date:   Tue, 31 Mar 2020 10:34:30 +0200
From:   Karel Zak <kzak@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>, dray@...hat.com,
        Miklos Szeredi <mszeredi@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Jeff Layton <jlayton@...hat.com>, Ian Kent <raven@...maw.net>,
        andres@...razel.de, keyrings@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lennart Poettering <lennart@...ttering.net>,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()

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@...ntu.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@...hat.com>
 http://karelzak.blogspot.com