From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <a3d8f244f00be51b2b9f80895be0be93@quanstro.net>
From: erik quanstrom <quanstro@quanstro.net>
Date: Tue,  1 Sep 2009 19:49:33 -0400
To: 9fans@9fans.net
In-Reply-To: <dd6fe68a0909011622l20f9e6abv2f3ebf75254b3e1e@mail.gmail.com>
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Subject: Re: [9fans] lowest valid stack address
Topicbox-Message-UUID: 5df5e50c-ead5-11e9-9d60-3106f5b1d025

On Tue Sep  1 19:24:33 EDT 2009, rsc@swtch.com wrote:
> I believe that the Stack line in /proc/$pid/segment
> tells you the lowest possible stack address, not
> the amount in use right now.  I looked in the kernel
> source and tried it in 9vx and it confirms my belief.
> I don't have a Plan 9 kernel to try it on right now,
> so maybe I'm wrong.
>
> Do you have two different /proc/$pid/segment
> with different lowest stack addresses on the Stack line?

evidently i misread /proc/$pid/segment.
and i guess this should have been obvious.
segments don't grow on demand.  but inside
a segment, you can have demand zero fill.

aside: from the overcommit vm discussion.
in http://9fans.net/archive/2000/06/634 rob
says that plan 9 doesn't overcommit vm.
what's the history here?

- erik

---
#include <u.h>
#include <libc.h>

enum {
	Stack	= 15*1024*1024,
};

char *runargv[] = {"/bin/sed", "1q", 0, 0};

void
run(char **argv)
{
	switch(fork()){
	case -1:
		sysfatal("fork: %r");
	case 0:
		exec(*argv, argv);
		sysfatal("exec: %r");
	default:
		waitpid();
	}
}

void
bigstack(void)
{
	char big[Stack];

	memset(big, 0, sizeof big);
	run(runargv);
}

void
main(void)
{
	char buf[64];

	snprint(buf, sizeof buf, "/proc/%ud/segment", getpid());
	runargv[2] = buf;

	run(runargv);
	bigstack();
	exits("");
}