Re: [PATCH v1 1/1] mm/vmalloc.c: Fix percpu free VM area search criteria

[sathyanarayanan kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 7/30/19 1:54 PM, Dave Hansen wrote:
> On 7/30/19 1:46 PM, Uladzislau Rezki wrote:
>>> +		/*
>>> +		 * If required width exeeds current VA block, move
>>> +		 * base downwards and then recheck.
>>> +		 */
>>> +		if (base + end > va->va_end) {
>>> +			base = pvm_determine_end_from_reverse(&va, align) - end;
>>> +			term_area = area;
>>> +			continue;
>>> +		}
>>> +
>>>   		/*
>>>   		 * If this VA does not fit, move base downwards and recheck.
>>>   		 */
>>> -		if (base + start < va->va_start || base + end > va->va_end) {
>>> +		if (base + start < va->va_start) {
>>>   			va = node_to_va(rb_prev(&va->rb_node));
>>>   			base = pvm_determine_end_from_reverse(&va, align) - end;
>>>   			term_area = area;
>>> -- 
>>> 2.21.0
>>>
>> I guess it is NUMA related issue, i mean when we have several
>> areas/sizes/offsets. Is that correct?
> I don't think NUMA has anything to do with it.  The vmalloc() area
> itself doesn't have any NUMA properties I can think of.  We don't, for
> instance, partition it into per-node areas that I know of.
>
> I did encounter this issue on a system with ~100 logical CPUs, which is
> a moderate amount these days.

I agree with Dave. I don't think this issue is related to NUMA. The 
problem here is about the logic we use to find appropriate vm_area that 
satisfies the offset and size requirements of pcpu memory allocator.

In my test case, I can reproduce this issue if we make request with 
offset (ffff000000) and size (600000).

>
-- 
Sathyanarayanan Kuppuswamy
Linux kernel developer