mm/memory_hotplug: maintain N_NORMAL_MEMORY during hotplug

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 most visible effect is that
/sys/devices/system/node/has_normal_memory does not report a node even
after that node has gained normal memory via hotplug.

Also, list_lru-based shrinkers can undercount objects on such a node
and may skip reclaim on that node entirely, which can lead to a higher
memory footprint than expected.

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.

Link: https://lkml.kernel.org/r/20260330035941.518186-1-hao.li@linux.dev
Fixes: 8d2882a8edb8 ("mm,memory_hotplug: remove status_change_nid_normal and update documentation")
Signed-off-by: Hao Li <hao.li@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index bc805029da517585bf89ea7adee769eeaf49b411..05a47953ef21b5a23859bad777ccbb1c27688862 100644 (file)
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1209,6 +1209,13 @@ int online_pages(unsigned long pfn, unsigned long nr_pages,
 
        if (node_arg.nid >= 0)
                node_set_state(nid, N_MEMORY);
+       /*
+        * Check whether we are adding normal memory to the node for the first
+        * time.
+        */
+       if (!node_state(nid, N_NORMAL_MEMORY) && zone_idx(zone) <= ZONE_NORMAL)
+               node_set_state(nid, N_NORMAL_MEMORY);
+
        if (need_zonelists_rebuild)
                build_all_zonelists(NULL);
 
@@ -1908,6 +1915,8 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
        unsigned long flags;
        char *reason;
        int ret;
+       unsigned long normal_pages = 0;
+       enum zone_type zt;
 
        /*
         * {on,off}lining is constrained to full memory sections (or more
@@ -2055,6 +2064,17 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages,
        /* reinitialise watermarks and update pcp limits */
        init_per_zone_wmark_min();
 
+       /*
+        * Check whether this operation removes the last normal memory from
+        * the node. We do this before clearing N_MEMORY to avoid the possible
+        * transient "!N_MEMORY && N_NORMAL_MEMORY" state.
+        */
+       if (zone_idx(zone) <= ZONE_NORMAL) {
+               for (zt = 0; zt <= ZONE_NORMAL; zt++)
+                       normal_pages += pgdat->node_zones[zt].present_pages;
+               if (!normal_pages)
+                       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.