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

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Date:   Wed, 7 Nov 2018 16:10:01 +0000
From:   Daniel Colascione <dancol@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, rppt@...ux.ibm.com,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] Document /proc/pid PID reuse behavior

On Wed, Nov 7, 2018 at 4:00 PM, Michal Hocko <mhocko@...nel.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@...nel.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.