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

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Message-ID: <20200811163905.GM1236603@ZenIV.linux.org.uk>
Date:   Tue, 11 Aug 2020 17:39:05 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        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 Tue, Aug 11, 2020 at 09:09:36AM -0700, Linus Torvalds wrote:
> On Tue, Aug 11, 2020 at 9:05 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Except that you suddenly see non-directory dentries get children.
> > And a lot of dcache-related logics needs to be changed if that
> > becomes possible.
> 
> Yeah, I think you'd basically need to associate a (dynamic)
> mount-point to that path when you start doing O_ALT. Or something.

Whee...  That's going to be non-workable for xattrs - fgetxattr()
needs to work after unlink().  And you'd obviously need to prevent
crossing into that sucker on normal lookups, which would add quite
a few interesting twists around the automount points.

I'm not saying it's not doable, but it won't be anywhere near
straightforward.  And API semantics questions are still there...