[PATCH] mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

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

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