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

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Date:   Tue, 31 Mar 2020 11:21:01 +0200
From:   Karel Zak <kzak@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     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,
        Christian Brauner <christian.brauner@...ntu.com>,
        keyrings@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()

On Mon, Mar 30, 2020 at 10:28:56PM +0200, Miklos Szeredi wrote:
> All this could be solved with a string key/value representation of the
> same data, with minimal performance loss on encoding/parsing.  The
> proposed fs interface[1] is one example of that, but I could also
> imagine a syscall based one too.

Yes, key/value is possible solution. The question is if we really 
need to add extra /sys-like filesystem to get key/value ;-) I can 
imagine key/value from FD based interface without open/read/close for
each attribute,

    fd = open("/mnt", O_PATH);
    fsinfo(fd, "propagation", buf, sizeof(buf));
    fsinfo(fd, "fstype", buf, sizeof(buf));
    close(fd);

why I need /mountfs/<id>/propagation and /mountfs/<id>/fstype to get
the same? It sounds like over-engineering without any extra bonus.

Anyway, if we have FD based interfaces like fsopen(), fsmount(),
open_tree() and move_mount() then it sounds strange that you cannot
use the FD to ask kernel for the mount node attributes and you need 
to open and read another /sys-like files. 

IMHO it would be nice that after open(/mnt, O_PATH) I can do whatever
with the mount point (umount, move, reconfigure, query, etc.). Please,
try to keep it simple and consistent ;-)

    Karel

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