Re: [PATCH] mm/memory_hotplug: factor out node_is_memoryless()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:

New issues:
- [Low] Unused function warnings when CONFIG_MEMORY_HOTREMOVE is disabled.

Pre-existing issues:
- [High] Data race on `node_spanned_pages` between ACPI CPU hot-remove and ZONE_DEVICE memory hotplug due to missing memory hotplug lock.
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commit 239875f97a26527846083c3d6ab82ee6b800ce0a
Author: Gregory Price <gourry@gourry.net>

mm/memory_hotplug: factor out node_is_memoryless()

A memoryless node neither spans present pages (populated or ZONE_DEVICE)
nor has an offline-but-added memory block still linked to it in sysfs.

try_offline_node() presently open-codes this memoryless check.

Pull that into a node_is_memoryless() helper and pull the existing
check_no_memblock_for_node_cb() helper ahead of the add/online path
so it's clearer what is happening here.

> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 226ab9cb078ad..d0e94057682af 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1491,6 +1491,36 @@ static int create_altmaps_and_memory_blocks(int nid, struct memory_group *group,
>  	return ret;
>  }
>  
> +static int check_no_memblock_for_node_cb(struct memory_block *mem, void *arg)
> +{
> +	int nid = *(int *)arg;
> +
> +	/*
> +	 * If a memory block belongs to multiple nodes, the stored nid is not
> +	 * reliable. However, such blocks are always online (e.g., cannot get
> +	 * offlined) and, therefore, are still spanned by the node.
> +	 */
> +	return mem->nid == nid ? -EEXIST : 0;
> +}
> +
> +/* Caller must hold the memory hotplug lock for this check. */
> +static bool node_is_memoryless(int nid)
> +{
> +	/*
> +	 * A node still spanning pages (especially ZONE_DEVICE) is not
> +	 * memoryless.  A node spans memory after move_pfn_range_to_zone(),
> +	 * e.g. once a memory block has been onlined.
> +	 */
> +	if (node_spanned_pages(nid))

[Severity: High]
This is a pre-existing issue and was not introduced by this patch, but can
this lockless read of node_spanned_pages() race with ZONE_DEVICE memory
hotplug?

The comment explicitly requires the memory hotplug lock, but the ACPI CPU
hot-remove path in drivers/acpi/acpi_processor.c:acpi_processor_post_eject()
calls try_offline_node() while holding only device_hotplug_lock():

drivers/acpi/acpi_processor.c:acpi_processor_post_eject() {
    ...
    cpus_write_unlock();
    cpu_maps_update_done();

    try_offline_node(cpu_to_node(pr->id));
}

Since ZONE_DEVICE memory hotplug (via memremap_pages()) modifies
node_spanned_pages under mem_hotplug_lock() without holding
device_hotplug_lock(), could this race result in a NUMA node with active
ZONE_DEVICE memory being erroneously offlined and unregistered from sysfs?

> +		return false;
> +	/*
> +	 * Offline memory blocks may not be spanned by the node yet, but they
> +	 * link to it in sysfs and can be onlined later, so the node is not
> +	 * memoryless while any remain.
> +	 */
> +	return !for_each_memory_block(&nid, check_no_memblock_for_node_cb);
> +}

[Severity: Low]
Does moving check_no_memblock_for_node_cb() and node_is_memoryless() outside
of the #ifdef CONFIG_MEMORY_HOTREMOVE block cause -Wunused-function warnings
when CONFIG_MEMORY_HOTREMOVE is disabled?

Their only caller, try_offline_node(), is conditionally compiled inside that
block, so these helpers would be unused in builds without it.

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260902195507.88655-1-gourry@gourry.net?part=1