linux-kernel - Re: thread stacks and strict vm overcommit accounting

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Date:	Thu, 15 Mar 2007 23:08:40 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Dan Aloni <da-x@...atomic.org>, linux-kernel@...r.kernel.org
Subject: Re: thread stacks and strict vm overcommit accounting

On Thu, 15 Mar 2007, Andrew Morton wrote:
> On Thu, 15 Mar 2007 23:33:43 +0000
> Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> 
> > > Stack RSS should certainly be included in Committed_AS,
> > > but RLIMIT_STACK merely limits how big the stack vma may grow to:
> > > at any moment the stack vma is probably very much smaller,
> > > and only its current size is accounted in Committed_AS.
> > 
> > With a typical size as a fuzz factor preaccounted in later kernels.
> 
> Where's that done?

I don't know what Alan is referring to there.

> 
> > > > > Is this the intended behaviour?
> > > > 
> > > > That sounds like a bug to me.
> > > 
> > > I'm suspecting it's an oddity rather than a bug.
> > 
> > It is intended behaviour.

Intended in the way the different stacks are implemented,
but odd enough for us to wonder at the difference.

> 
> Each instance of
> 
> main()
> {
> 	sleep(100);
> }
> 
> appears to increase Committed_AS by around 200kb.  But we've committed to
> providing it with 8MB for stack.
> 
> How come this is correct?

We've no more committed to providing each instance with 8MB of stack,
than we've committed to providing each instance with RLIMIT_AS of
address space.  The rlimits are limits, not commitments, surely?

Hugh
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