Re: [PATCH v2] Document /proc/pid PID reuse behavior

From: Daniel Colascione <dancol@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	rppt@linux.ibm.com, Tim Murray <timmurray@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Andrew Morton <akpm@linux-foundation.org>,
	Roman Gushchin <guro@fb.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
	Prashant Dhamdhere <pdhamdhe@redhat.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior
Date: Wed, 7 Nov 2018 16:10:01 +0000	[thread overview]
Message-ID: <CAKOZuevb-MED0QiL5RWJ0py=yt48=N7bPSq5bFqiwWcqN9L=Xg@mail.gmail.com> (raw)
In-Reply-To: <20181107160015.GI27423@dhcp22.suse.cz>

On Wed, Nov 7, 2018 at 4:00 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Wed 07-11-18 15:48:20, Daniel Colascione wrote:
>> On Tue, Nov 6, 2018 at 1:05 PM, Michal Hocko <mhocko@kernel.org> wrote:
>> > otherwise anybody could simply DoS the system
>> > by consuming all available pids.
>>
>> People can do that today using the instrument of terror widely known
>> as fork(2). The only thing standing between fork(2) and a full process
>> table is RLIMIT_NPROC.
>
> not really.

What else, besides memory consumption and (as you mention below)
cgroups? In practice, nobody uses RLIMIT_NPROC, so outside of various
container-y namespaced setups, avoidance of
system-DoS-through-PID-exhaustion isn't a pressing problem.

If you really do care about pid space depletion then you
> should use pid cgroup controller.

Or that, sure. But since cgroups are optional, the problem with the
core model remains. In general, if there's a problem X with the core
system API, and you can mitigate X by using a cgroup, X is still a
problem.