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    SubjectRe: [PATCH 20/20] honor r/w changes at do_remount() time
    On Tue, Jun 27, 2006 at 03:14:57PM -0700, Dave Hansen wrote:
    > Originally from: Herbert Poetzl <>
    > This is the core of the read-only bind mount patch set.
    > Note that this does _not_ add a "ro" option directly to
    > the bind mount operation. If you require such a mount,
    > you must first do the bind, then follow it up with a
    > 'mount -o remount,ro' operation.

    I guess the fundamental problem I have with that approach is that it's
    a cop-out - we just declare rw state of vfsmount independent from that
    of filesystem and add a "if a flag is set, act upon vfsmount".

    And yes, some of that does make sense. Fine, let's separate that
    stuff; but then we'd better decide what rw superblock *is*.

    We have a number of vfsmounts over given superblock. OK, some are
    "we don't even ask them to be r/w". Some are "we want them r/w, but
    don't actually use as such at the moment". Some are "pinned down for
    write now". And we do get logics for "can't make it r/o right now".

    But look - we have the _same_ logics for superblock itself. Only it's
    full of holes. And since you have rw states for those completely
    unrelated to those of vfsmount, we get a ridiculous situation - we
    *do* mark the moments when superblock becomes impossible to remount
    r/o and we even mostly get the moments when it ceases to be busy
    writing (unlinked-but-opened files are major exception). But we can't
    use that information.

    So "can we remount superblock ro?" turns into kinda-sorta duplicate of
    the same for vfsmounts, but it's racy as hell and bloody incomplete;
    we don't even get "if some vfsmount over it is busy writing, we won't
    remount r/o". Approximation is done, but that's it. E.g. mkdir() in
    progress does *not* stop remount of superblock r/o (it does prevent
    remount of vfsmount with your patchset).

    FWIW, I suspect that the root of the problem is that we confuse different
    states of filesystem. E.g. one obviously useful feature would be to have
    soft r/o - filesystem that is (from the driver POV) mounted readonly,
    but would get transparently switched r/w at the first request. And you
    have all vfsmount-side infrastructure for that, BTW. Add something like
    mechanism we use for expiry and you've got a very tasty feature for e.g.
    laptop users: e.g. userland asking to switch fs soft-ro every 15 minutes
    and if nobody had wanted it r/w since the last time, do the transition;
    if asked r/w again, r/w it goes on its own. IOW, there's more to it than
    one bit. And I'm talking about superblock state...

    BTW, it might be worth doing the following:
    * reintroduce the list of vfsmounts over given superblock (protected
    by vfsmount_lock)
    * keep ro flag separate from counter and split it in two.
    * all decrements are with atomic_dec_and_lock()
    * all increments are with atomic_add_unless() + spin_lock() +
    check flags + atomic_add_return() + possible spin_unlock
    * if writers count goes from non-zero to zero or vice versa
    increment/decrement superblock counter (number of vfsmounts that really
    want write access).
    * make the moments when i_nlink hits 0 bump the superblock writers
    count; drop it when such sucker gets freed on final iput.
    * kill the sodding "traverse the list of opened files" logics in
    remounting superblock r/o. Instead of that, grab spinlock, check writers
    count, bail out if non-zero, grab vfsmount_lock, traverse the list over
    superblock and set one of the flags, drop the locks and proceed.
    * when remounting superblock r/w, traverse the list and knock out
    the same flag.

    At least that way we'd get the majority of "can remount ro" logics right...

    Another fun issues:
    a) MS_REC handling with MS_BIND remounts (trivial)
    b) figuring out what (if anything) should be done with propagation
    when we have shared subtrees... (not trivial at all)
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