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

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Message-ID: <ada7i7so1o7.fsf@cisco.com>
Date:	Tue, 28 Oct 2008 14:22:16 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	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.

 > 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?

 - R.
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