linux-kernel - Re: [PATCH] regression: vmalloc easily fail.

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Message-ID: <20081028144255.05712d70@infradead.org>
Date:	Tue, 28 Oct 2008 14:42:55 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Avi Kivity <avi@...hat.com>, Glauber Costa <glommer@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	aliguori@...emonkey.ws, npiggin@...e.de,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Krzysztof Helt <krzysztof.h1@...zta.fm>
Subject: Re: [PATCH] regression: vmalloc easily fail.

On Tue, 28 Oct 2008 14:22:16 -0700
Roland Dreier <rdreier@...co.com> wrote:

>  > I'm guessing that the missing comment explains that this is
>  > intentional, to trap buffer overflows?
> 
> Actually, speaking of comments, it's interesting that
> __get_vm_area_node() -- which is called from vmalloc() -- does:
> 
> 	/*
> 	 * We always allocate a guard page.
> 	 */
> 	size += PAGE_SIZE;
> 
> 	va = alloc_vmap_area(size, align, start, end, node, gfp_mask);
> 
> and alloc_vmap_area() adds another PAGE_SIZE, as the original email
> pointed out:
> 
> 		while (addr + size >= first->va_start && addr + size
> <= vend) { addr = ALIGN(first->va_end + PAGE_SIZE, align);
> 
> I wonder if the double padding is causing a problem when things get
> too fragmented?

I suspect it's a case of off-by-one... ALIGN() might round down, and
the "+ (PAGE_SIZE-1)" was there to make it round up.
Except for that missing -1 ...

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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