linux-kernel - Re: [uml-devel] [PATCH 4/5] UML - Simplify helper stack handling

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Message-Id: <200707031728.36472.blaisorblade@yahoo.it>
Date:	Tue, 3 Jul 2007 17:28:30 +0200
From:	Blaisorblade <blaisorblade@...oo.it>
To:	user-mode-linux-devel@...ts.sourceforge.net
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jeff Dike <jdike@...toit.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [uml-devel] [PATCH 4/5] UML - Simplify helper stack handling

On giovedì 28 giugno 2007, Andrew Morton wrote:
> So I'm running the generic version of this on i386 with 8k stacks (below),
> with a quick LTP run.
>
> Holy cow, either we use a _lot_ of stack or these numbers are off:
>
> vmm:/home/akpm> dmesg -s 1000000|grep 'bytes left'
> khelper used greatest stack depth: 7176 bytes left
> khelper used greatest stack depth: 7064 bytes left
> khelper used greatest stack depth: 6840 bytes left
> khelper used greatest stack depth: 6812 bytes left
> hostname used greatest stack depth: 6636 bytes left
> uname used greatest stack depth: 6592 bytes left
> uname used greatest stack depth: 6284 bytes left
> hotplug used greatest stack depth: 5568 bytes left
> rpc.nfsd used greatest stack depth: 5136 bytes left
> chown02 used greatest stack depth: 4956 bytes left
> fchown01 used greatest stack depth: 4892 bytes left

> That's the sum of process stack and interrupt stack, but I doubt if this
> little box is using much interrupt stack space.
>
> No wonder people are still getting stack overflows with 4k stacks...

First, those numbers pretend to be _unused_ stack space.

Well, UML tends to use more stack space than the rest of kernel. Apart it has 
a bit more layering (even if less than in the past), we must use libc's 
function too, and they're not written to be executed on an 8k stack.

We've reimplemented libc's printf() in terms of kernel sprintf() because it 
used 32K of stack.
-- 
Inform me of my mistakes, so I can add them to my list!
Paolo Giarrusso, aka Blaisorblade
http://www.user-mode-linux.org/~blaisorblade

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