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    SubjectRe: [GIT PULL] notification tree - try 37!
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    [Adding linux-fsdevel here as well.]

    On Tuesday 17 August 2010 10:09:50 Andreas Gruenbacher wrote:
    > > > Q: What prevents the system from going out of memory when a listener
    > > > decides to stop reading events or simply can't keep up? There doesn't
    > > > seem to be a limit on the queue depth. Listeners currently need
    > > > CAP_SYS_ADMIN, but somehow limiting the queue depth and throttling when
    > > > things start to go bad still sounds like a reasonable thing to do,
    > > > right?
    > >
    > > It's an interesting question and obviously one that I've thought about.
    > > You remember when we talked previously I said the hardest part left was
    > > allowing non-root users to use the interface. It gets especially
    > > difficult when thinking about perm-events. I was specifically told not
    > > to timeout or drop those. But when dealing with non-root users using
    > > perm events? As for pure notification we can do something like inotify
    > > does quite easily.
    > >
    > > I'm not certain exactly what the best semantics are for non trusted
    > > users, so I didn't push any patches that way. Suggestions welcome :)
    >
    > The system will happily go OOM for trusted users and non-perm events if the
    > listener doesn't keep up, so some throttling, dropping, or both needs to
    > happen for non-perm events. This is the critical case. Doing what inotify
    > does (queue an overflow event and drop further events) seems to make sense
    > here.
    >
    > The situation with perm-events is less severe because the number of
    > outstanding perm events is bounded by the number of running processes.
    > This may be enough of a limit.
    >
    > I don't think we need to worry about perm-events for untrusted users. We
    > can start supporting some kinds of non-perm-events for untrusted users
    > later; this won't change the existing interface.

    Another case where fanotify fails to generate useful events is when a listener
    runs out of file descriptors; events will simply end up with fd == -EMFILE in
    that case. I don't think this behavior is useful; instead, reading from the
    fanotify file descriptor (he one returned by fanotify_init()) should fail to
    give the listener a chance to react.

    Andreas


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