linux-kernel - Re: Re : vm: weird behaviour when munmapping

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Date:	Fri, 17 Nov 2006 14:05:42 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	moreau francis <francis_moreau2000@...oo.fr>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re : vm: weird behaviour when munmapping

On Fri, 2006-11-17 at 12:50 +0000, moreau francis wrote:
> Peter Zijlstra wrote:
> > 
> > http://lwn.net/Kernel/LDD3/
> > 
> > Chapter 15. Section 'Virtual Memory Areas'.
> > 
> > Basically; vm_ops->open() is not called on the first vma. With this
> > munmap() you split the area in two, and it so happens the new vma is the
> > lower one.
> > 
> 
> since I did "munmap(0x2aaae000, 1024)" I would say that the the new vma
> is the _upper_ one.
> 
> lower vma: 0x2aaae000 -> 0x2aaaf000
> upper vma: 0x2aaaf000 -> 0x2aab2000

that is the remaining VMA, not the new one; we trigger this code:

	/* Does it split the last one? */
	last = find_vma(mm, end);
	if (last && end > last->vm_start) {
		int error = split_vma(mm, last, end, 1);
		if (error)
			return error;
	}

So, since its the last VMA that needs to be split (there is only one),
the new VMA is constructed before the old one. Like so:

  AAAAAAAAAAAAAAAAAAAAA
  BBBBAAAAAAAAAAAAAAAAA

Then you proceed closing, in this case the new one: B.


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