Re: [Patch V3 3/9] sgi-xp: Replace cpu_to_node() with cpu_to_mem() to support memoryless node

[Jiang Liu]

On 2015/8/20 14:36, Jiang Liu wrote:
> On 2015/8/20 8:02, David Rientjes wrote:
>> On Wed, 19 Aug 2015, Jiang Liu wrote:
>>
>>>> Why not simply fix build_zonelists_node() so that the __GFP_THISNODE 
>>>> zonelists are set up to reference the zones of cpu_to_mem() for memoryless 
>>>> nodes?
>>>>
>>>> It seems much better than checking and maintaining every __GFP_THISNODE 
>>>> user to determine if they are using a memoryless node or not.  I don't 
>>>> feel that this solution is maintainable in the longterm.
>>> Hi David,
>>> 	There are some usage cases, such as memory migration,
>>> expect the page allocator rejecting memory allocation requests
>>> if there is no memory on local node. So we have:
>>> 1) alloc_pages_node(cpu_to_node(), __GFP_THISNODE) to only allocate
>>> memory from local node.
>>> 2) alloc_pages_node(cpu_to_mem(), __GFP_THISNODE) to allocate memory
>>> from local node or from nearest node if local node is memoryless.
>>>
>>
>> Right, so do you think it would be better to make the default zonelists be 
>> setup so that cpu_to_node()->zonelists == cpu_to_mem()->zonelists and then 
>> individual callers that want to fail for memoryless nodes check 
>> populated_zone() themselves?
> Hi David,
> 	Great idea:) I think that means we are going to kill the
> concept of memoryless node, and we only need to specially handle
> a few callers who really care about whether there is memory on
> local node.
> 	Then I need some time to audit all usages of __GFP_THISNODE
> and update you whether it's doable.
Hi David,
	It seems that I'm too optimistic:(. After auditing all usages
of __GFP_THISNODE and reading Documentation/vm/numa again, I feel it
would be better to keep cpu_to_mem()/numa_mem_id(). It makes things
more clear if we follow rules:
1) cpu_to_node()/numa_node_id() for schedule domain
2) cpu_to_mem()/numa_mem_id() for memory management domain
3) alloc_pages_node(cpu_to_node(cpu), __GFP_THIS_NODE) for special
   usage cases.
   And it would be easier for maintenance than open-coded checking of
populated_zone() by using alloc_pages_node(cpu_to_node(cpu),
__GFP_THIS_NODE).
Thanks!
Gerry
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