linux-kernel - Re: [PATCH] Don't mlock guardpage if the stack is growing up

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Date:	Mon, 9 May 2011 15:07:41 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Cc:	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-ia64@...r.kernel.org
Subject: Re: [PATCH] Don't mlock guardpage if the stack is growing up

On Mon, May 9, 2011 at 8:57 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Hmm. One thing that strikes me is this problem also implies that the
> /proc/self/maps file is wrong for the GROWSUP case, isn't it?
>
> So I think we should not just apply your lock fix, but then *also*
> apply something like this:

Actually, I think we might be better off with something like this.

It makes a few more changes:

 - move the stack guard page checking in __get_user_pages() into the
rare case (ie we didn't find a page), since that's the only case we
care about (the thing about the guard page is that don't want to call
"handle_mm_fault()"). As a result, it's off any path where we can
possibly care about performance, so we might as well have a nice
helper function for both the grow-up and grow-down cases, instead of
trying to be clever and only look at the grow-down case for the first
page in the vma like you did in your patch.

   End result: simpler, more straightforward code.

 - Move the growsup/down helper functions to <linux/mm.h>, since the
/proc code really wants to use them too. That means that the
"vma_stack_continue()" function (which now got split up into two
cases, for the up/down cases) is now entirely just an internal helper
function - nobody else uses it, and the real interface are the
"stack_guard_page_xyz()"  functions. Renamed to be simpler.

 - changed that naming of those stack_guard_page functions to use
_start and _end instead of growsup/growsdown, since it actually takes
the start or the end of the page as the argument (to match the
semantics of the afore-mentioned helpers)

 - and finally, make /proc/<pid>/maps use these helpers for both the
up/down case, so now /proc/self/maps should work well for the growsup
case too.

Hmm?

The only oddish case is IA64 that actually has a stack that grows
*both* up and down. That means that I could make up a stack mapping
that has a single virtual page in it, that is both the start *and* the
end page. Now /proc/self/maps would actually show such a mapping with
"negative" size. That's interesting.

It would be easy enough to have a "if (end < start) end = start" there
for that case, but maybe it's actually interesting information.

Regardless, I'd like to hear whether this patch really does work on
PA-RISC and especially IA64. I think those are the only cases that
have a GROWSUP stack. And the IA64 case that supports both is the most
interesting, everybody else does just one or the other.

                    Linus

View attachment "patch.diff" of type "text/x-patch" (3827 bytes)