linux-kernel - Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set N

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Date: Fri, 16 Feb 2024 18:11:40 +0900
From: Byungchul Park <byungchul@...com>
To: Oscar Salvador <osalvador@...e.de>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kernel_team@...ynix.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set
 N_MEMORY

On Fri, Feb 16, 2024 at 08:52:30AM +0100, Oscar Salvador wrote:
> 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().

It seems that I got it wrong about the reason. Let me check it again and
share the reason.

Just in case, this patch is still definitely necessary tho.

	Byungchul