linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Message-ID: <CAJfpegsQF1aN4XJ_8j977rnQESxc=Kcn7Z2C+LnVDWXo4PKhTQ@mail.gmail.com>
Date:   Wed, 12 Aug 2020 17:13:14 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Karel Zak <kzak@...hat.com>, Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On Wed, Aug 12, 2020 at 5:08 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> 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?

Not allowed.

> 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?"

No.

>
> > > 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.

No.  The shared inode is not for lookup, just for the open file.

>  "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.

Why does it have to have a struct mount?  It does not have to use
dentry/mount based path lookup.

Thanks,
Miklos