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Date:   Tue, 11 Aug 2020 17:05:34 +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>,
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        Ian Kent <raven@themaw.net>,
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        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 Tue, Aug 11, 2020 at 08:20:24AM -0700, Linus Torvalds wrote:

> 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.
> 
> So you couldn't do it with _one_ pathname, because that is always
> fundamentally going to hit pathname lookup rules.
> 
> But if you start a new path lookup with new rules, that's fine.

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.

I agree that xattrs are garbage, but this approach won't be
a straightforward solution.  Can those suckers be passed to
...at() as starting points?  Can they be bound in namespace?
Can something be bound *on* them?  What do they have for inodes
and what maintains their inumbers (and st_dev, while we are at
it)?  Can _they_ have secondaries like that (sensu Swift)?
Is that a flat space, or can they be directories?

Only a part of the problems is implementation-related (and those are
not trivial at all); most the fun comes from semantics of those things.
And answers to the implementation questions are seriously dependent upon
that...