linux-kernel - Re: [2/3] mm: fix up some user-visible effects of the stack guard page

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Message-ID: <1282321974.12544.24.camel@zakaz.uk.xensource.com>
Date:	Fri, 20 Aug 2010 17:32:54 +0100
From:	Ian Campbell <ijc@...lion.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Greg KH <gregkh@...e.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [2/3] mm: fix up some user-visible effects of the stack guard
 page

On Fri, 2010-08-20 at 09:07 -0700, Linus Torvalds wrote:

> Actually, thinking some more about it, that may not be a good idea.
> Why? Simply because we may want to merge the vma's back together if
> you do munlock. And it won't (and mustn't) merge if the vm_flags
> differ in VM_GROWSDOWN.
> 
> So I do think we want to keep VM_GROWSDOWN (and VM_GROWSUP on PA-RISC)
> even across a vma split.

I naively hacked something together and it did seem to work, but I
shared your worries about merging.

> Of course, we could set a flag whether the vma really does have a
> guard page or not.

Bits in vma->vm_flags seems to be in rather short supply :-(

> That said, it does strike me as rather odd to do VM ops on partial
> stacks. What are you doing, exactly, to hit this?

I sent a contrived test program in my other mail.

The actual use is to mlock a buffer on the stack in order to pass it to
a Xen hypercall. The contract with the hypervisor is that the pages
passed to the hypercall must be mapped.

Ian.

-- 
Ian Campbell
Current Noise: Dinosaur Jr. - Green Mind

We can defeat gravity.  The problem is the paperwork involved.

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