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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