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

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Message-ID: <0c5610f128fa49fb9d8f7859e6f61b90@AcuMS.aculab.com>
Date:   Thu, 8 Nov 2018 12:02:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Martin Steigerwald' <martin@...htvoll.de>,
        Michal Hocko <mhocko@...nel.org>
CC:     Daniel Colascione <dancol@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "rppt@...ux.ibm.com" <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

From: Martin Steigerwald
> Sent: 07 November 2018 17:05
...
> Its not quite on-topic, but I am curious now: AFAIK PID limit is 16
> bits. Right? Could it be raised to 32 bits? I bet it would be a major
> change throughout different parts of the kernel.

It is probably 15 bits (since -ve pid numbers are used for process groups).

My guess is that userspace and the system call interface will handle 32bit
(signed) pid numbers.
(I don't remember 'linux emulation' being one of the emulations that
would truncate 32bit pids when one of the BDSs went to 32bit pids.)
The main problem will be that big numbers will mess up the alignment
of printouts from ps and top (etc).
This can be mitigated by only allocating 'big' numbers on systems
that have a lot of pids.
You also really want an O(1) allocator.

	David

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