Date: Tue, 28 Oct 2008 19:09:47 -0200
From: Glauber Costa <glommer@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, aliguori@codemonkey.ws,
       npiggin@suse.de, Jeremy Fitzhardinge <jeremy@goop.org>,
       Krzysztof Helt <krzysztof.h1@poczta.fm>
Subject: Re: [PATCH] regression: vmalloc easily fail.
Message-ID: <20081028210947.GA4269@poweredge.glommer>
References: <1225234513-3996-1-git-send-email-glommer@redhat.com> <49077E1A.5080105@redhat.com>
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On Tue, Oct 28, 2008 at 11:03:22PM +0200, Avi Kivity wrote:
> 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'm hoping Nick will comment on it. I might well be wrong.
but it nicely fixes the problem for me, and actually, you don't need 
"at least N/2 pages". The size of the allocations hardly matters, just
the amount of allocations we did. Since kvm does some small
vmalloc allocations, that may be the reason for we triggering it.


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