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

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Date:   Tue, 7 Dec 2021 16:09:39 +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 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 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.

-- 
Thanks,

David / dhildenb