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    SubjectRe: [PATCH] Reserve N process to root
    Date
    Kyle Moffett wrote:
    > Please don't trim CC lists
    >
    > On Oct 11, 2007, at 17:02:37, Al Boldi wrote:
    > > David Newall wrote:
    > >> Valdis.Kletnieks@vt.edu wrote:
    > >>> What David meant was that "root will always have a slot" doesn't
    > >>> *actually* help unless you *also* have a way to actually *spawn*
    > >>> such a process. In order to do the ps, kill, and so on that you
    > >>> need to recover, you need to already have either a root shell
    > >>> available, or a way to *get* a root shell that doesn't rely on a
    > >>> non-root process (so /bin/su doesn't help here).
    > >>
    > >> That's right, although it's worse than that. You need to have a
    > >> process with CAP_SYS_ADMIN. If root processes normally have that
    > >> capability then the reserved slots may well disappear before you
    > >> notice a problem. If root processes normally don't have it, then
    > >> you need to guarantee that one is already running.
    > >
    > > I once posted a patch to handle this DoS, but, as usual, it wasn't
    > > accepted. Go figure...
    >
    > This isn't really necessary any more with the new CFS scheduler. If
    > you want to prevent excess memory usage then you limit memory usage,
    > not process count, so just set the system max process count to
    > something absurdly high and leave the user counts down at the maximum
    > a user might run. Then as long as the sum of the user processes is
    > less than the max number of processes (which you just set absurdly
    > high or unlimited), you may still log in. With the per-user
    > scheduling enabled CFS allows you to run an optimistically-real-time
    > game as one user and several thousand busy-loops as another user and
    > get almost picture perfect 50% CPU distribution between the users.
    > To me that seems a much better DoS-prevention system than limits
    > which don't scale based on how many people are requesting resources.

    You have a point, and resource-controllers can probably control DoS a lot
    better, but the they also incur more overhead. Think of this "lockout
    prevention" patch as a near zero overhead safety valve.


    Thanks!

    --
    Al

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