linux-kernel - Re: [PATCH] Add /proc/pid

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Message-ID: <20181125225533.GB30242@amd>
Date:   Sun, 25 Nov 2018 23:55:33 +0100
From:   Pavel Machek <pavel@....cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Daniel Colascione <dancol@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Primiano Tucci <primiano@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, rostedt@...dmis.org,
        tglx@...utronix.de, mingo@...nel.org, linux@...inikbrodowski.net,
        pasha.tatashin@...cle.com, jpoimboe@...hat.com,
        ard.biesheuvel@...aro.org, Michal Hocko <mhocko@...e.com>,
        David Howells <dhowells@...hat.com>, ktsanaktsidis@...desk.com,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] Add /proc/pid_generation

On Wed 2018-11-21 18:06:33, Matthew Wilcox wrote:
> On Wed, Nov 21, 2018 at 12:38:20PM -0800, Daniel Colascione wrote:
> > On Wed, Nov 21, 2018 at 12:31 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Wed, Nov 21, 2018 at 12:14:44PM -0800, Daniel Colascione wrote:
> > > > This change adds a per-pid-namespace 64-bit generation number,
> > > > incremented on PID rollover, and exposes it via a new proc file
> > > > /proc/pid_generation. By examining this file before and after /proc
> > > > enumeration, user code can detect the potential reuse of a PID and
> > > > restart the task enumeration process, repeating until it gets a
> > > > coherent snapshot.
> > > >
> > > > PID rollover ought to be rare, so in practice, scan repetitions will
> > > > be rare.
> > >
> > > Then why does it need to be 64-bit?
> > 
> > [Resending because of accidental HTML. I really need to switch to a
> > better email client.]
> > 
> > Because 64 bits is enough for anyone. :-) A u64 is big enough that
> > we'll never observe an overflow on a running system, and PID
> > namespaces are rare enough that we won't miss the four extra bytes we
> > use by upgrading from a u32.  And after reading about some security
> > problems caused by too-clever handling of 32-bit rollover, I'd rather
> > the code be obviously correct than save a trivial amount of space.
> 
> I don't think you understand how big 4 billion is.  If it happens once a
> second, it will take 136 years for a 2^32 count to roll over.  How often
> does a PID roll over happen?

Well, the cost of 64-bit vs. 32-bit is really small here... I'd go
with 64bits. If you have 1000 CPUs, rollovers may be faster..

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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