Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 11 Aug 2020 17:39:05 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        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)
Message-ID: <20200811163905.GM1236603@ZenIV.linux.org.uk>
References: <1842689.1596468469@warthog.procyon.org.uk>
 <1845353.1596469795@warthog.procyon.org.uk>
 <CAJfpegunY3fuxh486x9ysKtXbhTE0745ZCVHcaqs9Gww9RV2CQ@mail.gmail.com>
 <ac1f5e3406abc0af4cd08d818fe920a202a67586.camel@themaw.net>
 <CAJfpegu8omNZ613tLgUY7ukLV131tt7owR+JJ346Kombt79N0A@mail.gmail.com>
 <CAJfpegtNP8rQSS4Z14Ja4x-TOnejdhDRTsmmDD-Cccy2pkfVVw@mail.gmail.com>
 <20200811135419.GA1263716@miu.piliscsaba.redhat.com>
 <CAHk-=wjzLmMRf=QG-n+1HnxWCx4KTQn9+OhVvUSJ=ZCQd6Y1WA@mail.gmail.com>
 <20200811160534.GL1236603@ZenIV.linux.org.uk>
 <CAHk-=wgHZig-+dXQeW4pPEjdYsrq=3bgc+vUhwiT2Ox4ipLHwg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wgHZig-+dXQeW4pPEjdYsrq=3bgc+vUhwiT2Ox4ipLHwg@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@zeniv.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...