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

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Date:	Fri, 20 Aug 2010 17:35:01 +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 17:32 +0100, Ian Campbell wrote:

> 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 :-( 

On the other hand the VMA merging is just an optimisation, isn't it?

-- 
Ian Campbell

Many people are desperately looking for some wise advice which will
recommend that they do what they want to do.

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