Re: DOS by unprivileged user

From: Michal Hocko
Date: Sun Apr 22 2018 - 20:27:53 EST

On Sun 22-04-18 10:43:00, vcaputo@xxxxxxxxxxx wrote:
> On Sun, Apr 22, 2018 at 12:16:54PM +0200, Pavel Machek wrote:
> > On Thu 2018-04-19 21:13:35, Ferry Toth wrote:
> > > It appears any ordinary user can easily create a DOS on linux.
> > >
> > > One sure way to reproduce this is to open gitk on the linux kernel repo
> > > (SIC) on a machine with 8GB RAM 16 GB swap on a HDD with btrfs and quad core
> > > + hyperthreading. But I will be easy enough to get the same effect with more
> > > RAM, other fs etc.
> >
> > You may want to disable swap.
> >
> I run without swap on my laptops, and still observe long periods of
> thrashing on the road towards OOM. What seems to occur is the active
> file-backed mappings of executables/libraries become a sort of swap
> area, repeatedly being discarded and faulted back in as the context
> switches occur.
> If there's any good way to prevent this, I'd like to know.

I am afraid there is none yet. Johannes had some ground work for
page cache trashing detection
but there was no version of the patchseries for quite some time and
there was no integration into the oom detection which would be
non-trivial as well.

I realize this sucks. But the reality is that this is far from trivial
to resolve without introducing pre-mature OOM killer invocations.
Michal Hocko