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

[Joshua Hahn <joshua.hahnjy@gmail.com>]

On Fri, 27 Mar 2026 15:44:18 +0100 "Vlastimil Babka (SUSE)" <vbabka@kernel.org> 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
> 
> Note in practice it's unlikely that a node would hotplug normal memory,
> start using it, and then manage to successfully hotremove it, due to
> unmovable allocations. Most likely only ZONE_MOVABLE memory can get hotremoved.

Hello Vlastimil! I hope you are doing well.

Yup, makes sense : -)

> > 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. 
> 
> Ignoring nodes that have normal memory, just because it was hotplugged, will
> result also just in some form of inefficiency, or can the consequences be worse?

Aaaahh yeah, seems like ignoring the hotplugged normal memory would be a bigger
problem. struct zswap_entries allocated from slab allocator on hotplugged
nodes would just get ignored by shrink_memcg, seems like a much bigger
issue than iterating through extra nodes :P thanks for calling this out.

> >> 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.
> 
> But commit 8d2882a8edb8 didn't introduce the current state, or did it?

I don't mean to speak on Hao's behalf, but as far as I can tell I think this
would have been the commit to introduce the state, since 
node_states_check_changes_online would set status_change_nid_normal to nid,
then nodes_states_set_node would come around and set it to N_NORMAL_MEMORY.
Maybe I'm missing something? :0

Thanks for the reply, Vlastimil. I hope you have a great day : -)
Joshua