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Date: Fri, 16 Feb 2024 08:52:30 +0100
From: Oscar Salvador <osalvador@suse.de>
To: Byungchul Park <byungchul@sk.com>
Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, vschneid@redhat.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	kernel_team@skhynix.com, akpm@linux-foundation.org
Subject: Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set
 N_MEMORY
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 <20240216070754.GB32626@system.software.com>
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On Fri, Feb 16, 2024 at 04:07:54PM +0900, Byungchul Park wrote:
> For normal numa nodes, node_data[] is initialized at alloc_node_data(),
> but it's not for memoryless node. However, the node *gets onlined* at
> init_cpu_to_node().
> 
> Let's look at back free_area_init(). free_area_init_node() will be called
> with node_data[] not set yet, because it's already *onlined*. So
> ->zone_pgdat cannot be initialized properly in the path you mentioned.

I am might be missing something., so bear with me.

free_area_init() gets called before init_cpu_to_node() does.
free_area_init_node() gets called on every possible node.

free_area_init_node then() does

 pg_data_t *pgdat = NODE_DATA(nid);,

and then we call free_area_init_core().

free_area_init_core() does

 free_area_init_core() does
  zone_init_internals()

which ends up doing zone->zone_pgdat = NODE_DATA(nid);

If node_data[] was not set at all, we would already blow up when doing
the first

  for_each_node()
    pgdat = NODE_DATA(nid);
    free_area_init_node(nid);

back in free_area_init().


-- 
Oscar Salvador
SUSE Labs