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    SubjectRe: [PATCH] netpoll can lock up on low memory.
    On Sat, Aug 06, 2005 at 09:45:03AM +0200, Ingo Molnar wrote:
    > * Andi Kleen <> wrote:
    > > On Fri, Aug 05, 2005 at 01:01:57PM -0700, Matt Mackall wrote:
    > > > The netpoll philosophy is to assume that its traffic is an absolute
    > > > priority - it is better to potentially hang trying to deliver a panic
    > > > message than to give up and crash silently.
    > >
    > > That would be ok if netpoll was only used to deliver panics. But it is
    > > not. It delivers all messages, and you cannot hang the kernel during
    > > that. Actually even for panics it is wrong, because often it is more
    > > important to reboot in a panic than (with a panic timeout) to actually
    > > deliver the panic. That's needed e.g. in a failover cluster.
    > without going into the merits of this discussion, reliable failover
    > clusters must include (and do include) an external ability to cut power.
    > No amount of in-kernel logic will prevent the kernel from hanging, given
    > a bad enough kernel bug.

    Ok, true, but we should do a best effort.

    > So the right question is not 'can we prevent the kernel from hanging,
    > ever' (we cannot), but 'which change makes it less likely for the kernel
    > to hang'. (and, obviously: assuming all other kernel components are
    > functioning per specification, netpoll itself most not hang :-)
    > even a plain printk to VGA can hang in certain kernel crashes. Netpoll
    > is more complex and thus has more exposure to hangs. E.g. netpoll relies
    > on the network driver to correctly recycle skbs within a bound amount of
    > time. If the network driver leaks skbs, it's game over for netpoll.

    I don't think we even need to think about such rare cases,
    until the easy cases ("everything hangs when the cable is pulled")
    are not fixed.

    > [ i'd prefer a hang over nondeterministic behavior, and e.g. losing
    > console messages is sure nondeterministic behavior. What if the
    > console message is "WARNING: the box has just been broken into"? ]

    That just makes netconsole useless in production. If it causes frequenet
    hangs people will not use it.

    > we could do one thing (see the patch below): i think it would be useful
    > to fill up the netlogging skb queue straight at initialization time.
    > Especially if netpoll is used for dumping alone, the system might not be
    > in a situation to fill up the queue at the point of crash, so better be
    > a bit more prepared and keep the pipeline filled.

    You're solving a completely different issue here?


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