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    SubjectRe: [PATCH 3/3] livepatch: add shadow variable sample program

    > > > I often wonder whether it's really a good idea to even allow the
    > > > unloading of patch modules at all. It adds complexity to the livepatch
    > > > code. Is it worth it? I don't have an answer but I'd be interested in
    > > > other people's opinion.
    > >
    > > I could imagine a situation when a livepatch causes, for example,
    > > performance, problems on a server because of the redirection
    > > to the new code. Then it might be handy to disable the patch
    > > and ftrace handlers completely.
    >
    > Fair enough, though it sounds theoretical. It would be good to know
    > we're supporting actual real world use cases.

    We distribute cumulative "replace_all" patches at SUSE. replace_all means
    that all previous patches are reverted in the process of application. All
    livepatch modules with zero refcount are removed. This keeps a number of
    loaded modules low and system's state well defined, which is always a good
    thing, because a customer might run into problems and we'd have to debug
    it.

    It is true that it is a limitation too. Especially for state changes and
    data structure modifications. Sometimes it is easy to patch a system, but
    impossible to unpatch it. Because we don't have a consistency on a state
    level, only on a task/process level. But I perceive this also as an
    advantage. I have to always know what a livepatch does exactly and I
    discovered couple of problems just because I had to think about unloading
    of modules.

    Miroslav

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