mm/memory_hotplug: initialize memmap of !ZONE_DEVICE with PageOffline() instead of PageReserved()

We currently initialize the memmap such that PG_reserved is set and the
refcount of the page is 1.  In virtio-mem code, we have to manually clear
that PG_reserved flag to make memory offlining with partially hotplugged
memory blocks possible: has_unmovable_pages() would otherwise bail out on
such pages.

We want to avoid PG_reserved where possible and move to typed pages
instead.  Further, we want to further enlighten memory offlining code
about PG_offline: offline pages in an online memory section.  One example
is handling managed page count adjustments in a cleaner way during memory
offlining.

So let's initialize the pages with PG_offline instead of PG_reserved. 
generic_online_page()->__free_pages_core() will now clear that flag before
handing that memory to the buddy.

Note that the page refcount is still 1 and would forbid offlining of such
memory except when special care is take during GOING_OFFLINE as currently
only implemented by virtio-mem.

With this change, we can now get non-PageReserved() pages in the XEN
balloon list.  From what I can tell, that can already happen via
decrease_reservation(), so that should be fine.

HV-balloon should not really observe a change: partial online memory
blocks still cannot get surprise-offlined, because the refcount of these
PageOffline() pages is 1.

Update virtio-mem, HV-balloon and XEN-balloon code to be aware that
hotplugged pages are now PageOffline() instead of PageReserved() before
they are handed over to the buddy.

We'll leave the ZONE_DEVICE case alone for now.

Note that self-hosted vmemmap pages will no longer be marked as
reserved.  This matches ordinary vmemmap pages allocated from the buddy
during memory hotplug.  Now, really only vmemmap pages allocated from
memblock during early boot will be marked reserved.  Existing
PageReserved() checks seem to be handling all relevant cases correctly
even after this change.

Link: https://lkml.kernel.org/r/20240607090939.89524-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Oscar Salvador <osalvador@suse.de> [generic memory-hotplug bits]
Cc: Alexander Potapenko <glider@google.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eugenio Pérez <eperezma@redhat.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Marco Elver <elver@google.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 10 insertions, 6 deletions

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index e5aafed2b9ce..f9dc9c2c9469 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -734,7 +734,7 @@ static inline void section_taint_zone_device(unsigned long pfn)
 /*
  * Associate the pfn range with the given zone, initializing the memmaps
  * and resizing the pgdat/zone data to span the added pages. After this
- * call, all affected pages are PG_reserved.
+ * call, all affected pages are PageOffline().
  *
  * All aligned pageblocks are initialized to the specified migratetype
  * (usually MIGRATE_MOVABLE). Besides setting the migratetype, no related
@@ -1101,8 +1101,12 @@ int mhp_init_memmap_on_memory(unsigned long pfn, unsigned long nr_pages,
 
 	move_pfn_range_to_zone(zone, pfn, nr_pages, NULL, MIGRATE_UNMOVABLE);
 
-	for (i = 0; i < nr_pages; i++)
-		SetPageVmemmapSelfHosted(pfn_to_page(pfn + i));
+	for (i = 0; i < nr_pages; i++) {
+		struct page *page = pfn_to_page(pfn + i);
+
+		__ClearPageOffline(page);
+		SetPageVmemmapSelfHosted(page);
+	}
 
 	/*
 	 * It might be that the vmemmap_pages fully span sections. If that is
@@ -1960,9 +1964,9 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 	 * Don't allow to offline memory blocks that contain holes.
 	 * Consequently, memory blocks with holes can never get onlined
 	 * via the hotplug path - online_pages() - as hotplugged memory has
-	 * no holes. This way, we e.g., don't have to worry about marking
-	 * memory holes PG_reserved, don't need pfn_valid() checks, and can
-	 * avoid using walk_system_ram_range() later.
+	 * no holes. This way, we don't have to worry about memory holes,
+	 * don't need pfn_valid() checks, and can avoid using
+	 * walk_system_ram_range() later.
 	 */
 	walk_system_ram_range(start_pfn, nr_pages, &system_ram_pages,
 			      count_system_ram_pages_cb);