Re: [PATCH v2 2/2] mm: migrate: Allocate the node_demotion structure dynamically

["Huang, Ying" <ying.huang@intel.com>]

Baolin Wang <baolin.wang@linux.alibaba.com> writes:

> On 2021/11/11 16:51, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
>> 
>>> For the worst case (MAX_NUMNODES=1024), the node_demotion structure can
>>> consume 32k bytes, which appears too large, so we can change to allocate
>>> node_demotion dynamically at initialization time. Meanwhile allocating
>>> the target demotion nodes array dynamically to select a suitable size
>>> according to the MAX_NUMNODES.
>>>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>   mm/migrate.c | 38 +++++++++++++++++++++++++++++---------
>>>   1 file changed, 29 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 126e9e6..0145b38 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1152,10 +1152,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>>   #define DEFAULT_DEMOTION_TARGET_NODES 15
>>>   struct demotion_nodes {
>>>   	unsigned short nr;
>>> -	short nodes[DEFAULT_DEMOTION_TARGET_NODES];
>>> +	short nodes[];
>>>   };
>>>   -static struct demotion_nodes node_demotion[MAX_NUMNODES]
>>> __read_mostly;
>>> +static struct demotion_nodes *node_demotion[MAX_NUMNODES] __read_mostly;
>>> +static unsigned short target_nodes_max;
>> I think we can use something as below,
>>    #if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
>>    #define DEMOTION_TARGET_NODES   (MAX_NUMNODES - 1)
>>    #else
>>    #define DEMOTION_TARGET_NODES   DEFAULT_DEMOTION_TARGET_NODES
>>    #endif
>
> Yes, looks better.
>
>>    static struct demotion_nodes *node_demotion;
>> Then we can allocate nr_node_ids * sizeof(struct demotion_nodes) for
>> node_demotion.
>
> Yeah, this is simple. The reason I want to declare the structure like
> "struct demotion_nodes *node_demotion[MAX_NUMNODES]" is that, we can 
> validate the non-possible nodes which are invalid to demote memory,
> and in case the node_demotion[nid] is failed to be allocated which can
> be validated, though this is unlikely.

In case allocation failure, we can still check "node_demotion == NULL".

Best Regards,
Huang, Ying

> However, I agree with you to
> keep things simple now and can be merged into patch 1. Will do in next 
> version. Thanks.