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Date:   Wed, 12 Aug 2020 16:08:07 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Jann Horn <jannh@google.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Andy Lutomirski <luto@amacapital.net>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Karel Zak <kzak@redhat.com>, Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
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On Wed, Aug 12, 2020 at 04:46:20PM +0200, Miklos Szeredi wrote:

> > "Can those suckers be passed to
> > ...at() as starting points?
> 
> No.

Lovely.  And what of fchdir() to those?  Are they all non-directories?
Because the starting point of ...at() can be simulated that way...

> >  Can they be bound in namespace?
> 
> No.
> 
> > Can something be bound *on* them?
> 
> No.
> 
> >  What do they have for inodes
> > and what maintains their inumbers (and st_dev, while we are at
> > it)?
> 
> Irrelevant.  Can be some anon dev + shared inode.
> 
> The only attribute of an attribute that I can think of that makes
> sense would be st_size, but even that is probably unimportant.
> 
> >  Can _they_ have secondaries like that (sensu Swift)?
> 
> Reference?

http://www.online-literature.com/swift/3515/
	So, naturalists observe, a flea
	Has smaller fleas that on him prey;
	And these have smaller still to bite 'em,
	And so proceed ad infinitum.
of course ;-)
IOW, can the things in those trees have secondary trees on them, etc.?
Not "will they have it in your originally intended use?" - "do we need
the architecture of the entire thing to be capable to deal with that?"

> > Is that a flat space, or can they be directories?"
> 
> Yes it has a directory tree.   But you can't mkdir, rename, link,
> symlink, etc on anything in there.

That kills the "shared inode" part - you'll get deadlocks from
hell that way.  "Can't mkdir" doesn't save you from that.  BTW,
what of unlink()?  If the tree shape is not a hardwired constant,
you get to decide how it's initially populated...

Next: what will that tree be attached to?  As in, "what's the parent
of its root"?  And while we are at it, what will be the struct mount
used with those - same as the original file, something different
attached to it, something created on the fly for each pathwalk and
lazy-umounted?  And see above re fchdir() - if they can be directories,
it's very much in the game.