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    SubjectRe: Should we split the network filesystem setup into two phases?
    On Thu, Aug 16, 2018 at 12:06:06AM -0500, Eric W. Biederman wrote:

    > So I don't think we can completely abandon the option for filesystems
    > to always create a new filesystem instance when mount(8) is called.

    Huh? If filesystem wants to create a new instance on each ->mount(),
    it can bloody well do so. Quite a few do - if that fs can handle
    that, more power to it.

    The problem is what to do with filesystems that *can't* do that.
    You really, really can't have two ext4 (or xfs, etc.) instances over
    the same device at the same time. Cache coherency, locking, etc.
    will kill you.

    And that's not to mention the joy of defining the semantics of
    having the same ext4 mounted with two logs at the same time ;-)

    I've seen "reject unless the options are compatible/identical/whatever",
    but that ignores the real problem with existing policy. It's *NOT*
    "I've mounted this and got an existing instance with non-matching
    options". That's a minor annoyance (and back when that decision
    had been made, mount(2) was very definitly root-only). The real
    problem is different and much worse - it's remount.

    I have asked to mount something and it had already been mounted,
    with identical options. OK, so what happens if I do mount -o remount
    on my instance? *IF* we are operating in the "only sysadmin can
    mount new filesystems", it's not a big deal - there are already
    lots of ways you can shoot yourself in the foot and mount(2) is
    certainly a powerful one. But if we get to "Joe R. Luser can do
    it in his container", we have a big problem.

    Decision back then had been mostly for usability reasons - it was
    back in 2001 (well before the containermania, userns or anything
    of that sort) and it was more about "how many hoops does one have
    to jump through to get something mounted, assuming the sanity of
    sysadmin doing that?". If *anything* like userns had been a concern
    back then, it probably would've been different. However, it's
    17 years too late and if anyone has a functional TARDIS, I can
    easily think of better uses for it...

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