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

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Date:	Tue, 28 Oct 2008 23:03:22 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	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.

Glauber Costa wrote:
> Commit db64fe02258f1507e13fe5212a989922323685ce broke
> KVM (the symptom) for me. The cause is that vmalloc
> allocations fail, despite of the fact that /proc/meminfo
> shows plenty of vmalloc space available.
>
> After some investigation, it seems to me that the current
> way to compute the next addr in the rb-tree transversal
> leaves a spare page between each allocation. After a few
> allocations, regardless of their size, we run out of vmalloc
> space.
>
>  
>  		while (addr + size >= first->va_start && addr + size <= vend) {
> -			addr = ALIGN(first->va_end + PAGE_SIZE, align);
> +			addr = ALIGN(first->va_end, align);
>  
>  			n = rb_next(&first->rb_node);
>  			if (n)
>   

I'm guessing that the missing comment explains that this is intentional, 
to trap buffer overflows?

(okay that was a cheap shot.  I don't comment nearly enough either)

Even if you leave a page between allocations, I don't see how you can 
fail a one page allocation, unless you've allocated at least N/2 pages 
(where N is the size of the vmalloc space in pages).

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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