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

[David Hildenbrand (Arm)]

On 3/27/26 15:38, Joshua Hahn wrote:
> On Fri, 27 Mar 2026 20:42:47 +0800 Hao Li <hao.li@xxxxxxxxx> wrote:
> 
> Hello Hao,
> 
> I hope you are doing well, thank you for the patch!
> 
>> 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.
> 
> The second part feels more important than the second part, doing a quick
> glance through the code I can see a few N_NORMAL_MEMORY iterators that
> are in some hot paths like shrink_memcg. Iterating over nodes that don't
> contain any NORMAL memory seems like an inefficiency rather than a bug
> though. 
> 
>> 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.
>>
>> Signed-off-by: Hao Li <hao.li@xxxxxxxxx>
>> ---
>>
>> This patch also prepares for a subsequent SLUB change that makes
>> can_free_to_pcs() rely on N_NORMAL_MEMORY to decide whether an object can be
>> freed to the sheaf.
>>
>> ---
>>  mm/memory_hotplug.c | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index bc805029da51..5498744aa1f1 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -1155,6 +1155,7 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>>  	int need_zonelists_rebuild = 0;
>>  	unsigned long flags;
>>  	int ret;
>> +	bool need_set_normal_memory = false;
>>  
>>  	/*
>>  	 * {on,off}lining is constrained to full memory sections (or more
>> @@ -1180,6 +1181,9 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>>  		if (ret)
>>  			goto failed_addition;
>>  	}
>> +	/* Adding normal memory to the node for the first time */
>> +	if (!node_state(nid, N_NORMAL_MEMORY) && zone_idx(zone) <= ZONE_NORMAL)
>> +		need_set_normal_memory = true;
>>  
>>  	ret = memory_notify(MEM_GOING_ONLINE, &mem_arg);
>>  	ret = notifier_to_errno(ret);
>> @@ -1209,6 +1213,8 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
>>  
>>  	if (node_arg.nid >= 0)
>>  		node_set_state(nid, N_MEMORY);
>> +	if (need_set_normal_memory)
>> +		node_set_state(nid, N_NORMAL_MEMORY);
>>  	if (need_zonelists_rebuild)
>>  		build_all_zonelists(NULL);
> 
> Do we need the flag here? As far as I can tell, we can just skip this and just
> directly check whether this is the first normal memory we are adding to the
> node here and set the bit. Then we can remove the flag and the extraneous
> check. We won't do any notifier work so I think it should be OK.

I assume you simply mean:


if (zone_idx(zone) <= ZONE_NORMAL && !node_state(nid, N_NORMAL_MEMORY))
	node_set_state(nid, N_NORMAL_MEMORY);

That looks m uch better!

> 
>> @@ -1908,6 +1914,9 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>>  	unsigned long flags;
>>  	char *reason;
>>  	int ret;
>> +	bool need_clear_normal_memory = false;
>> +	unsigned long node_normal_pages = 0;
>> +	enum zone_type zt;
>>  
>>  	/*
>>  	 * {on,off}lining is constrained to full memory sections (or more
>> @@ -1977,6 +1986,13 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>>  			goto failed_removal_isolated;
>>  		}
>>  	}
>> +	/*
>> +	 * Check whether this operation removes the node's last normal memory.
>> +	 */
>> +	for (zt = 0; zt <= ZONE_NORMAL; zt++)
>> +		node_normal_pages += pgdat->node_zones[zt].present_pages;
>> +	if (nr_pages >= node_normal_pages && zone_idx(zone) <= ZONE_NORMAL)
>> +		need_clear_normal_memory = true;
>>  
>>  	ret = memory_notify(MEM_GOING_OFFLINE, &mem_arg);
>>  	ret = notifier_to_errno(ret);
>> @@ -2055,6 +2071,12 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
>>  	/* reinitialise watermarks and update pcp limits */
>>  	init_per_zone_wmark_min();
> 
> Same here, couldn't we just iterate through the paegs here and accumulate
> node_normal_pages and clear the memory here? We can get rid of the bool and
> also keep node_normal_pages defined inside an if (zone_idx(zone) <= ZONE_NORMAL)
> check as well.

Right, all we have to do is check for remaining present_pages in one of
the nodes IIUC.

-- 
Cheers,

David