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    SubjectRe: [GIT PULL] Detaching mounts on unlink for 3.15
    Al Viro <viro@ZenIV.linux.org.uk> writes:

    > Have you tried to profile something like umount -l on a large mount tree?
    > You variant causes a shitstorm of
    > schedule work
    > switch to workqueue
    > do actual fs shutdown
    > wake umount(8) up
    > get through wait_for_completion()
    > for every bleeding vfsmount in there. And no, it's *not* guaranteed to
    > be dominated by fs shutdown time. Here's the case where it definitely
    > won't be:
    > mkdir /tmp/a
    > mount --rbind / /tmp/a
    > umount -l /tmp/a
    >
    > All vfsmounts involved are killed off with no fs shutdown. And that's
    > *not* a rare case - exit of the last process in namespace is very likely
    > to look that way too.
    >
    > That's far too heavy.

    The typically system has about 12 mounts and no mount namespaces.
    Making umount of any kind a rare case.

    Al my apologies for not picking your favorite solution and in choosing
    not to audit every call to mntput in the tree this week I have made an
    uncommon case slower.

    These bug fixes are important, the code is correct, and it is
    technically straight forward to remove the wait in mntput in most
    cases.

    I am very frustrated that when given lots of opportunities to look at
    and comment on my code it is hard to get you to engage and comment
    unless I send a pull request to Linus.

    This last objection really looks to be a case of perfection getting in
    the way of the good.

    Eric



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