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

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Message-ID: <CAJfpegtWai+5Tzxi1_G+R2wEZz0q66uaOFndNE0YEQSDjq0f_A@mail.gmail.com>
Date:   Tue, 11 Aug 2020 17:30:32 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, 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 5:20 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> [ I missed the beginning of this discussion, so maybe this was already
> suggested ]
>
> On Tue, Aug 11, 2020 at 6:54 AM Miklos Szeredi <miklos@...redi.hu> wrote:
> >
> > >
> > > E.g.
> > >   openat(AT_FDCWD, "foo/bar//mnt/info", O_RDONLY | O_ALT);
> >
> > Proof of concept patch and test program below.
>
> I don't think this works for the reasons Al says, but a slight
> modification might.
>
> IOW, if you do something more along the lines of
>
>        fd = open(""foo/bar", O_PATH);
>        metadatafd = openat(fd, "metadataname", O_ALT);
>
> it might be workable.

That would have been my backup suggestion, in case the unified
namespace doesn't work out.

I wouldn't think the normal lookup rules really get in the way if we
explicitly enable alternative path lookup with a flag.  The rules just
need to be documented.

What's the disadvantage of doing it with a single lookup WITH an enabling flag?

It's definitely not going to break anything, so no backward
compatibility issues whatsoever.

Thanks,
Miklos