linux-kernel - Re: DOS by unprivileged user

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Date:   Sun, 22 Apr 2018 18:27:38 -0600
From:   Michal Hocko <mhocko@...nel.org>
To:     vcaputo@...garu.com
Cc:     Pavel Machek <pavel@....cz>, Ferry Toth <ftoth@...fort.nl>,
        linux-kernel@...r.kernel.org
Subject: Re: DOS by unprivileged user

On Sun 22-04-18 10:43:00, vcaputo@...garu.com 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 https://marc.info/?i=20170727153010.23347-1-hannes%40cmpxchg.org
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
SUSE Labs