linux-kernel - Re: [PATCH v3] mm: fix panic in __alloc

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Date:   Tue, 7 Dec 2021 16:34:30 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Alexey Makhalov <amakhalov@...are.com>,
        Dennis Zhou <dennis@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v3] mm: fix panic in __alloc_pages

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);



> 
> 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.
Let's concentrate on preallocating the pgdat and fixing the issue at
hand. One step at a time please.


>> I agree to prealloc the pgdat, I don't think we should actually set the
>> nodes online. Node onlining/offlining should be done when we do have
>> actual CPUs/memory populated.
> 
> If we keep the offline/online node state notion we are not solving an
> important aspect of the problem - confusing api.

I don't think it's that confusing. Just like we do have online and
offline CPUs. Or online and offline memory blocks. Similarly, a node is
either online or offline.

-- 
Thanks,

David / dhildenb