Re: [V5 PATCH 05/26] node_states: introduce N_MEMORY

[Wen Congyang <wency@cn.fujitsu.com>]

At 10/30/2012 04:46 AM, David Rientjes Wrote:
> On Mon, 29 Oct 2012, Lai Jiangshan wrote:
> 
>> We have N_NORMAL_MEMORY for standing for the nodes that have normal memory with
>> zone_type <= ZONE_NORMAL.
>>
>> And we have N_HIGH_MEMORY for standing for the nodes that have normal or high
>> memory.
>>
> 
> (In other words, all memory.)
> 
>> But we don't have any word to stand for the nodes that have *any* memory.
>>
> 
> It's N_HIGH_MEMORY, or at least it's supposed to be.  Is there a problem 
> where the bit isn't getting set for a node with memory?
> 
>> A)	But this reusing is bad for *readability*. Because the name
>> 	N_HIGH_MEMORY just stands for high or normal:
>>
>> A.example 1)
>> 	mem_cgroup_nr_lru_pages():
>> 		for_each_node_state(nid, N_HIGH_MEMORY)
>>
>> 	The user will be confused(why this function just counts for high or
>> 	normal memory node? does it counts for ZONE_MOVABLE's lru pages?)
>> 	until someone else tell them N_HIGH_MEMORY is reused to stand for
>> 	nodes that have any memory.
>>
>> A.cont) If we introduce N_MEMORY, we can reduce this confusing
>> 	AND make the code more clearly:
>>
>> A.example 2) mm/page_cgroup.c use N_HIGH_MEMORY twice:
>>
>> 	One is in page_cgroup_init(void):
>> 		for_each_node_state(nid, N_HIGH_MEMORY) {
>>
>> 	It means if the node have memory, we will allocate page_cgroup map for
>> 	the node. We should use N_MEMORY instead here to gaim more clearly.
>>
>> 	The second using is in alloc_page_cgroup():
>> 		if (node_state(nid, N_HIGH_MEMORY))
>> 			addr = vzalloc_node(size, nid);
>>
>> 	It means if the node has high or normal memory that can be allocated
>> 	from kernel. We should keep N_HIGH_MEMORY here, and it will be better
>> 	if the "any memory" semantic of N_HIGH_MEMORY is removed.
>>
>> B)	This reusing is out-dated if we introduce MOVABLE-dedicated node.
>> 	The MOVABLE-dedicated node should not appear in
>> 	node_stats[N_HIGH_MEMORY] nor node_stats[N_NORMAL_MEMORY],
>> 	because MOVABLE-dedicated node has no high or normal memory.
>>
>> 	In x86_64, N_HIGH_MEMORY=N_NORMAL_MEMORY, if a MOVABLE-dedicated node
>> 	is in node_stats[N_HIGH_MEMORY], it is also means it is in
>> 	node_stats[N_NORMAL_MEMORY], it causes SLUB wrong.
>>
>> 	The slub uses
>> 		for_each_node_state(nid, N_NORMAL_MEMORY)
>> 	and creates kmem_cache_node for MOVABLE-dedicated node and cause problem.
>>
>> In one word, we need a N_MEMORY. We just intrude it as an alias to
>> N_HIGH_MEMORY and fix all im-proper usages of N_HIGH_MEMORY in late patches.
>>
> 
> If this is really that problematic (and it appears it's not given that 
> there are many use cases of it and people tend to get it right), then why 
> not simply rename N_HIGH_MEMORY instead of introducing yet another 
> nodemask to the equation?

The reason is that we need a node which only contains movable memory. This
feature is very important for node hotplug. So we will add a new nodemask
for movable memory. N_MEMORY contains movable memory but N_HIGH_MEMORY
doesn't contain it.

Thanks
Wen Congyang

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