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SubjectRe: A true story of a crash.
On Sat, 15 Aug 1998, Albert D. Cahalan wrote:
> Our current situation is bad. At the very least, Rik van Riel's process
> killer will not be worse than what we already have. I expect that most
> people will find it much better than merely "not worse" though.
>
> > It would be better and simpler to let the user or admin decide what to
> > kill. Instead of killing a process, we should put it to sleep.
>
> End result: 100% memory use, 100% idle, all processes stopped.

That depends on implementation. Of course, if you let every last page get
used before doing anything, you're stuck.

> > If the machine has overextended itself, we're probably swapping like mad
> > already. It's hammered. We're not getting anything done. We don't need
> > efficiency anymore. We want recovery without loosing in-process work.
>
> Not possible.

Gee that was fast.

> > For example, let's put each process, that asks for a page that we can't
> > give, to sleep (from do_no_page?). This would be a special sleep in that
> > it doesn't wakeup until we return to a certain threshold of free memory.
> > What would happen is that it's pages would age and get thrown out.
>
> Thrown out? You must mean that literally, since there may be no more swap.
> The process will be really messed up if you send pages to /dev/null.

No, I was referring to text pages. They don't use swap. Sorry.

> Hostile users can just use a daemon to grab the last bit of memory
> that you reserved for root. (finger, telnet, whatever)
>
> With the overcommit, random events could be enough to eat up all
> the reserved space.

Like I said, there is definitely the admin factor here. This is not a
countermeasure for any kind of attack. I'm just pointing out another
option besides SIGBUS. If a production machine's kernel suddenly kills
off your main app because you mistakenly underestimated resources, you may
want it to wait until you have a chance to give it more resources
instead of just killing it off and making you start over. ext2fs reserves
a bit of space for the admin. Perhaps a bit of swap should be reserved
also. The admin now has _MORE_ options.

-Matt


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