Re: [RFC v2 4/4] mm: fallback for offline nodes in alloc_pages_node

[Vlastimil Babka]

On 24.7.2015 21:54, David Rientjes wrote:
> On Fri, 24 Jul 2015, Christoph Lameter wrote:
> 
>> On Fri, 24 Jul 2015, Vlastimil Babka wrote:
>>
>>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
>>> index 531c72d..104a027 100644
>>> --- a/include/linux/gfp.h
>>> +++ b/include/linux/gfp.h
>>> @@ -321,8 +321,12 @@ static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
>>>  						unsigned int order)
>>>  {
>>>  	/* Unknown node is current (or closest) node */
>>> -	if (nid == NUMA_NO_NODE)
>>> +	if (nid == NUMA_NO_NODE) {
>>>  		nid = numa_mem_id();
>>> +	} else if (!node_online(nid)) {
>>> +		VM_WARN_ON(!node_online(nid));
>>> +		nid = numa_mem_id();
>>> +	}
>>
>> I would think you would only want this for debugging purposes. The
>> overwhelming majority of hardware out there has no memory
>> onlining/offlining capability after all and this adds the overhead to each
>> call to alloc_pages_node.
>>
>> Make this dependo n CONFIG_VM_DEBUG or some such thing?
>>
> 
> Yeah, the suggestion was for VM_WARN_ON() in the conditional, but the 
> placement has changed somewhat because of the new __alloc_pages_node().  I 
> think
> 
> 	else if (VM_WARN_ON(!node_online(nid)))
> 		nid = numa_mem_id();
> 
> should be fine since it only triggers for CONFIG_DEBUG_VM.

Um, so on your original suggestion I thought that you assumed that the condition
inside VM_WARN_ON is evaluated regardless of CONFIG_DEBUG_VM, it just will or
will not generate a warning. Which is how BUG_ON works, but VM_WARN_ON (and
VM_BUG_ON) doesn't. IIUC VM_WARN_ON() with !CONFIG_DEBUG_VM will always be false.
Because I didn't think you would suggest the "nid = numa_mem_id()" for
!node_online(nid) fixup would happen only for CONFIG_DEBUG_VM kernels. But it
seems that you do suggest that? I would understand if the fixup (correcting an
offline node to some that's online) was done regardless of DEBUG_VM, and
DEBUG_VM just switched between silent and noisy fixup. But having a debug option
alter the outcome seems wrong?
Am I correct that passing an offline node is not fatal, just the zonelist will
be empty and the allocation will fail? Now without DEBUG_VM it would silently
fail, and with DEBUG_VM it would warn, but succeed on another node.

So either we do fixup regardless of DEBUG_VM, or drop this patch, as the
VM_WARN_ON(!node_online(nid)) is already done in __alloc_pages_node() thanks to
patch 2/4?
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