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SubjectRe: file metadata via fs API (was: [GIT PULL] Filesystem Information)
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...

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