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

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Message-ID: <20181120091829.GD16916@amd>
Date:   Tue, 20 Nov 2018 10:18:29 +0100
From:   Pavel Machek <pavel@....cz>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Daniel Colascione <dancol@...gle.com>,
        linux-kernel@...r.kernel.org, rppt@...ux.ibm.com,
        timmurray@...gle.com, joelaf@...gle.com, surenb@...gle.com,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "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 Tue 2018-11-20 10:05:21, Vlastimil Babka wrote:
> On 11/19/18 11:54 AM, Pavel Machek wrote:
> > On Mon 2018-11-05 13:22:05, Daniel Colascione wrote:
> >> State explicitly that holding a /proc/pid file descriptor open does
> >> not reserve the PID. Also note that in the event of PID reuse, these
> >> open file descriptors refer to the old, now-dead process, and not the
> >> new one that happens to be named the same numeric PID.
> >>
> >> Signed-off-by: Daniel Colascione <dancol@...gle.com>
> >> ---
> >>  Documentation/filesystems/proc.txt | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> Moved paragraphed to start of /proc/pid section; added signed-off-by.
> >>
> >> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> >> index 12a5e6e693b6..0b14460f721d 100644
> >> --- a/Documentation/filesystems/proc.txt
> >> +++ b/Documentation/filesystems/proc.txt
> >> @@ -125,6 +125,13 @@ process running on the system, which is named after the process ID (PID).
> >>  The link  self  points  to  the  process reading the file system. Each process
> >>  subdirectory has the entries listed in Table 1-1.
> >>  
> >> +Note that an open a file descriptor to /proc/<pid> or to any of its
> >> +contained files or subdirectories does not prevent <pid> being reused
> >> +for some other process in the event that <pid> exits. Operations on
> > 
> > "does not" -> "may not"?
> > 
> > We want to leave this unspecified, so that we can change it in future.
> 
> Why can't the documentation describe the current implementation, and
> change in the future if the implementation changes? I doubt somebody

Documentation should describe "contract" between kernel and userspace.

> would ever rely on the pid being reused while having the descriptor
> open. How would that make sense?

I agree this is corner space, but users might be surprised that
keeping FDs of /proc/pid/X would lead to PID space exhaustion, for
example.

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

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