linux-kernel - Re: [PATCH] guard page for stacks that grow upwards

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Message-ID: <AANLkTi=-JRUFH08TYNhB7TyN=YscnNbqwtZ_BwcwKJPS@mail.gmail.com>
Date:	Tue, 24 Aug 2010 10:51:12 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
	linux-ia64@...r.kernel.org
Subject: Re: [PATCH] guard page for stacks that grow upwards

Hmm. Looking at the patch a bit more..

On Tue, Aug 24, 2010 at 9:31 AM, Luck, Tony <tony.luck@...el.com> wrote:
>
> +#if defined(CONFIG_STACK_GROWSUP) || defined(CONFIG_IA64)
> +       else if (vma->vm_flags & VM_GROWSUP) {
> +               address = PAGE_ALIGN(address + 1);
> +               if (address == vma->vm_end) {

So I react to two things:

 - that "else" looks wrong. At least conceptually, both could happen -
the code should be entirely able to handle a segment that expands both
ways (which is something that ia64 could do: stack one way, register
spills etc other, all in just one happy vma). I guess we don't set it
up that way now, but the "else" just annoys my sense of aesthetics.
It's an extra four letters that don't add value - just takes it away.

 - The "address = PAGE_ALIGN(address + 1);" thing is just ugly.
Wouldn't it be nicer to just move the earlier

  address &= PAGE_MASK

back outside the conditionals (and keep the original conditional the
way it was - you only changed it for that bogus "else" case anyway),
and then do

  if (address + PAGE_SIZE == vma->vm_end)

rather than have "PAGE_ALIGN(address + 1)" as yet another way of
aligning the address just right.

No?

                              Linus
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