linux-kernel - Re: [Patch 1/1] init: Provide a kernel start parameter to increase pid

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Date:	Sun, 25 Apr 2010 10:15:07 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Pavel Machek <pavel@....cz>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <gregkh@...e.de>,
	Rik van Riel <riel@...hat.com>,
	John Stoffel <john@...ffel.org>, Hedi Berriche <hedi@....com>,
	Mike Travis <travis@....com>, Ingo Molnar <mingo@...e.hu>,
	Jack Steiner <steiner@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robin Holt <holt@....com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Patch 1/1] init: Provide a kernel start parameter to increase
 pid_max v2

On Sun, 25 Apr 2010, Pavel Machek wrote:
> 
> Issue with max_pids is that it can break userspace, right?

Iirc, some _really_ old code used 'short' for pid_t, and we wanted to be 
really safe when we raised the limits. 

I seriously doubt we need to worry about old binaries like that on any 16+ 
CPU machines, though.

The other issue is just the size of the pidmap[] array. Instead of walking 
all the processes to see "is this pid in use" (like I think the original 
Linux kernel did), we have a bitmap of used pids. When you raise pid_max, 
that bitmap obviously still needs to be big enough. Right now we allocate 
that statically (rather than growing it dynamically), so we end up having 
a _hard_ limit of PID_MAX_LIMIT too.

On 32-bit, I think that still maximum limit ends up being basically 32767. 
So again, on a _legacy_ system, you end up being limited in the number of 
pid_t entries.

				Linus
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