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

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Message-ID: <AANLkTim1ABgiwscmaTZxeO9FxB4FacUNe0ahzUctQkn6@mail.gmail.com>
Date:	Fri, 20 Aug 2010 09:07:14 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ian Campbell <ijc@...lion.org.uk>
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>
Subject: Re: [2/3] mm: fix up some user-visible effects of the stack guard page

On Fri, Aug 20, 2010 at 8:54 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Aug 20, 2010 at 5:54 AM, Ian Campbell <ijc@...lion.org.uk> wrote:
>>
>> Since we have split the original VMA into 3, shouldn't only the bottom
>> one still have VM_GROWSDOWN set? (how can the top two grow down with the
>> bottom one in the way?) Certainly it seems wrong to enforce a guard page
>> on anything but the bottom VMA (which is what appears to be happening).
>
> Yes, it does seem like we should teach vma splitting to remove
> VM_GROWSDOWN on all but the lowest mapping.

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.

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

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

                         Linus
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