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

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Date:   Tue, 20 Nov 2018 09:48:27 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Pavel Machek <pavel@....cz>, Vlastimil Babka <vbabka@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mike Rapoport <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>,
        "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, Nov 20, 2018 at 9:39 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Nov 20, 2018 at 10:18:29AM +0100, Pavel Machek wrote:
> > > 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.
>
> We have a limit on the number of FDs a process can have open for a reason.
> Well, for many reasons.

And the typical limit is too low. (I've seen people clamp it to 1024
for some reason.) A file descriptor is just a handle to a kernel
resource. All kernel resources held on behalf of applications need
*some* kind of management interface. File descriptors provide a
consistent and uniform instance of such a management interface. Unless
there's a very good reason, nobody should be using non-FD handles for
kernel resource management. A low default FD table size limit is not
an example of one of these good reasons, not when we can raise FD
table size limit. In general, the software projects should not have to
put up with ugly workarounds for limitations they impose on
themselves.