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Date: Fri, 16 Feb 2024 16:07:54 +0900
From: Byungchul Park <byungchul@sk.com>
To: Oscar Salvador <osalvador@suse.de>
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
Message-ID: <20240216070754.GB32626@system.software.com>
References: <20240214035355.18335-1-byungchul@sk.com>
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On Wed, Feb 14, 2024 at 10:13:57PM +0100, Oscar Salvador wrote:
> On Wed, Feb 14, 2024 at 12:53:55PM +0900, Byungchul Park wrote:
> > While running qemu with a configuration where some CPUs don't have their
> > local memory and with a kernel numa balancing on, the following oops has
> > been observed. It's because of null pointers of ->zone_pgdat of zones of
> > those nodes that are not initialized at booting time. So should avoid
> > nodes not set N_MEMORY from getting promoted.
> 
> Looking at free_area_init(), we call free_area_init_node() for each node
> found on the system.
> And free_area_init_node()->free_area_init_core() inits all zones
> belonging to the system via zone_init_internals().

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.

	Byungchul

> Now, I am not saying the check is wrong because we obviously do not want
> migrate memory to a memoryless node, but I am confused as to where
> we are crashing.
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs