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

[Hao Li <hao.li@linux.dev>]

On Fri, Mar 27, 2026 at 07:38:23AM -0700, Joshua Hahn wrote:
> On Fri, 27 Mar 2026 20:42:47 +0800 Hao Li <hao.li@linux.dev> wrote:
> 
> Hello Hao,
> 
> I hope you are doing well, thank you for the patch!

Hi Joshua,

Thanks! Hope you are too.

> 
> > 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

I'm guessing you mean the first part.

> 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. 

Yes, I agree.

> 
> > 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@linux.dev>
> > ---
> > 
> > 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.

Exactly, this is a good point!

> 
> > @@ -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. 

Yes, the same to online_pages(), thanks for this idea.

> 
> > +	/*
> > +	 * Clear N_NORMAL_MEMORY first to avoid the transient state
> > +	 * "!N_MEMORY && N_NORMAL_MEMORY".
> > +	 */
> > +	if (need_clear_normal_memory)
> > +		node_clear_state(node, N_NORMAL_MEMORY);
> >  	/*
> >  	 * Make sure to mark the node as memory-less before rebuilding the zone
> >  	 * list. Otherwise this node would still appear in the fallback lists.
> > -- 
> > 2.50.1
> > 
> > 
> 
> Thank you for the patch. The rest looks good to me! Have a great day : -)
> Joshua

Thanks for the review!

-- 
Thanks,
Hao