Re: [PATCH v2] mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

[David Hildenbrand (Arm)]

On 3/30/26 05:57, Hao Li wrote:
> N_NORMAL_MEMORY is initialized from zone population at boot, but memory
> hotplug currently only updates N_MEMORY. As a result, a node that gains
> normal memory via hotplug can remain invisible to users iterating over
> N_NORMAL_MEMORY, while a node that loses its last normal memory can stay
> incorrectly marked as such.
> 
> Restore N_NORMAL_MEMORY maintenance directly in online_pages() and
> offline_pages(). Set the bit when a node that currently lacks normal
> memory onlines pages into a zone <= ZONE_NORMAL, and clear it when
> offlining removes the last present pages from zones <= ZONE_NORMAL.
> 
> This restores the intended semantics without bringing back the old
> status_change_nid_normal notifier plumbing which was removed in
> 8d2882a8edb8.
> 
> Current users that benefit include list_lru, zswap, nfsd filecache,
> hugetlb_cgroup, and has_normal_memory sysfs reporting.
> 
> Fixes: 8d2882a8edb8 ("mm,memory_hotplug: remove status_change_nid_normal and update documentation")

Agreed on CCing stable.

> Signed-off-by: Hao Li <hao.li@xxxxxxxxx>
> ---
> Changes: simplify the code. (Thanks Joshua and David)
> ---
>  mm/memory_hotplug.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index bc805029da51..05a47953ef21 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1209,6 +1209,13 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>  
>  	if (node_arg.nid >= 0)
>  		node_set_state(nid, N_MEMORY);
> +	/*
> +	 * Check whether we are adding normal memory to the node for the first
> +	 * time.
> +	 */
> +	if (!node_state(nid, N_NORMAL_MEMORY) && zone_idx(zone) <= ZONE_NORMAL)
> +		node_set_state(nid, N_NORMAL_MEMORY);
> +
>  	if (need_zonelists_rebuild)
>  		build_all_zonelists(NULL);
>  
> @@ -1908,6 +1915,8 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  	unsigned long flags;
>  	char *reason;
>  	int ret;
> +	unsigned long normal_pages = 0;
> +	enum zone_type zt;
>  
>  	/*
>  	 * {on,off}lining is constrained to full memory sections (or more
> @@ -2055,6 +2064,17 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>  	/* reinitialise watermarks and update pcp limits */
>  	init_per_zone_wmark_min();
>  
> +	/*
> +	 * Check whether this operation removes the last normal memory from
> +	 * the node. We do this before clearing N_MEMORY to avoid the possible
> +	 * transient "!N_MEMORY && N_NORMAL_MEMORY" state.
> +	 */
> +	if (zone_idx(zone) <= ZONE_NORMAL) {
> +		for (zt = 0; zt <= ZONE_NORMAL; zt++)
> +			normal_pages += pgdat->node_zones[zt].present_pages;
> +		if (!normal_pages)
> +			node_clear_state(node, N_NORMAL_MEMORY);
> +	}

Was wondering whether we could just do an early exit, like:


for (zt = 0; zt <= ZONE_NORMAL; zt++) {
	if (pgdat->node_zones[zt].present_pages)
		break;
}
if (zt > ZONE_NORMAL)
	node_clear_state(node, N_NORMAL_MEMORY);

But ends up being harder to digest (incl. all variants I tried :) ), so

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>


-- 
Cheers,

David