[PATCH] mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

[PATCH] mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

[Hao Li]

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.

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);
 
@@ -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();
 
+	/*
+	 * 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

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

[Joshua Hahn]

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.

> @@ -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. 

> +	/*
> +	 * 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

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

[Vlastimil Babka (SUSE)]

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.

> 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?

>> 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?

>> 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.
> 
>> @@ -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. 
> 
>> +	/*
>> +	 * 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

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

[David Hildenbrand (Arm)]

On 3/27/26 15:44, Vlastimil Babka (SUSE) 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.

Yes, hotunplug of all normal memory of a node is unlikely. Possible, but
nothing to optimize for.

-- 
Cheers,

David

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

[Hao Li]

On Fri, Mar 27, 2026 at 04:22:52PM +0100, David Hildenbrand (Arm) wrote:
> On 3/27/26 15:44, Vlastimil Babka (SUSE) 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.
> 
> Yes, hotunplug of all normal memory of a node is unlikely. Possible, but
> nothing to optimize for.

Yes, I agree that fully hot-unplugging the last normal-memory node is probably
uncommon.

That said, once we restore the set side, clearing it feels like the symmetric
counterpart as well. If we leave the clear side as-is, it seems like it could
be a bit confusing for others later on.

-- 
Thanks,
Hao

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

[Joshua Hahn]

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

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

[Hao Li]

On Fri, Mar 27, 2026 at 09:35:54AM -0700, Joshua Hahn wrote:
> 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 : -)

Yes, indeed.

> 
> > > 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.

Yes, make sense to me. It could be more serious than just inefficiency. The
shrinkers might even fail to visit reclaimable objects on that node.

> 
> > >> 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 this analysis. I was thinking the same. N_NORMAL_MEMORY is no longer
updated after this commit.

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

-- 
Thanks,
Hao

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@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!

> 
>> @@ -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

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

[Hao Li]

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

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

[Hao Li]

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