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

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

On Fri, Mar 27, 2026 at 04:28:53PM +0100, David Hildenbrand (Arm) wrote:
> On 3/27/26 15:38, 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!
> > 
> >> 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@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.
> 
> 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!

Yes, this approach is elegant! Thanks.

> 
> > 
> >> @@ -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.

Yes, we can simply sum up the remaining present_pages and check whether it
comes out to 0. That feels more straightforward and direct.

-- 
Thanks,
Hao