Re: [PATCH] regression: vmalloc easily fail.

[Glauber Costa <glommer@redhat.com>]

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.