Re: [PATCH v3] mm: fix panic in __alloc_pages

[David Hildenbrand <david@redhat.com>]

On 07.12.21 16:56, Michal Hocko wrote:
> On Tue 07-12-21 16:34:30, David Hildenbrand wrote:
>> On 07.12.21 16:29, Michal Hocko wrote:
>>> On Tue 07-12-21 16:09:39, David Hildenbrand wrote:
>>>> On 07.12.21 14:23, Michal Hocko wrote:
>>>>> On Tue 07-12-21 13:28:31, David Hildenbrand wrote:
>>>>> [...]
>>>>>> But maybe I am missing something important regarding online vs. offline
>>>>>> nodes that your patch changes?
>>>>>
>>>>> I am relying on alloc_node_data setting the node online. But if we are
>>>>> to change the call to arch_alloc_node_data then the patch needs to be
>>>>> more involved. Here is what I have right now. If this happens to be the
>>>>> right way then there is some additional work to sync up with the hotplug
>>>>> code.
>>>>>
>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>> index c5952749ad40..a296e934ad2f 100644
>>>>> --- a/mm/page_alloc.c
>>>>> +++ b/mm/page_alloc.c
>>>>> @@ -8032,8 +8032,23 @@ void __init free_area_init(unsigned long *max_zone_pfn)
>>>>>  	/* Initialise every node */
>>>>>  	mminit_verify_pageflags_layout();
>>>>>  	setup_nr_node_ids();
>>>>> -	for_each_online_node(nid) {
>>>>> -		pg_data_t *pgdat = NODE_DATA(nid);
>>>>> +	for_each_node(nid) {
>>>>> +		pg_data_t *pgdat;
>>>>> +
>>>>> +		if (!node_online(nid)) {
>>>>> +			pr_warn("Node %d uninitialized by the platform. Please report with memory map.\n", nid);
>>>>> +			pgdat = arch_alloc_nodedata(nid);
>>>>> +			pgdat->per_cpu_nodestats = alloc_percpu(struct per_cpu_nodestat);
>>>>> +			arch_refresh_nodedata(nid, pgdat);
>>>>> +			node_set_online(nid);
>>>>
>>>> Setting all possible nodes online might result in quite some QE noice,
>>>> because all these nodes will then be visible in the sysfs and
>>>> try_offline_nodes() is essentially for the trash.
>>>
>>> I am not sure I follow. I believe sysfs will not get populate because I
>>> do not call register_one_node.
>>
>> arch/x86/kernel/topology.c:topology_init()
>>
>> for_each_online_node(i)
>> 	register_one_node(i);
> 
> Right you are.
>  
>>> You are right that try_offline_nodes will be reduce which is good imho.
>>> More changes will be possible (hopefully to drop some ugly code) on top
>>> of this change (or any other that achieves that there are no NULL pgdat
>>> for possible nodes).
>>>
>>
>> No to exposing actually offline nodes to user space via sysfs.
> 
> Why is that a problem with the sysfs for non-populated nodes?
> 

https://lore.kernel.org/linuxppc-dev/20200428093836.27190-1-srikar@linux.vnet.ibm.com/t/

Contains some points -- certainly nothing unfixable but it clearly shows
that users expect only nodes with actual memory and cpus to be online --
that's why we export the possible+online state to user space. My point
is to be careful with such drastic changes and do one step at a time.

I think preallocation of the pgdat is a reasonable thing to have without
changing user-space visible semantics or even in-kernel semantics.

-- 
Thanks,

David / dhildenb