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>

3 files changed, 21 insertions, 11 deletions

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index 0e7427c2baf5..c38dcdfcb914 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -683,9 +683,8 @@ static void hv_page_online_one(struct hv_hotadd_state *has, struct page *pg)
 		if (!PageOffline(pg))
 			__SetPageOffline(pg);
 		return;
-	}
-	if (PageOffline(pg))
-		__ClearPageOffline(pg);
+	} else if (!PageOffline(pg))
+		return;
 
 	/* This frame is currently backed; online the page. */
 	generic_online_page(pg, 0);
diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index a3857bacc844..b90df29621c8 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -1146,12 +1146,16 @@ static void virtio_mem_set_fake_offline(unsigned long pfn,
 	for (; nr_pages--; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 
-		__SetPageOffline(page);
-		if (!onlined) {
+		if (!onlined)
+			/*
+			 * Pages that have not been onlined yet were initialized
+			 * to PageOffline(). Remember that we have to route them
+			 * through generic_online_page().
+			 */
 			SetPageDirty(page);
-			/* FIXME: remove after cleanups */
-			ClearPageReserved(page);
-		}
+		else
+			__SetPageOffline(page);
+		VM_WARN_ON_ONCE(!PageOffline(page));
 	}
 	page_offline_end();
 }
@@ -1166,9 +1170,11 @@ static void virtio_mem_clear_fake_offline(unsigned long pfn,
 	for (; nr_pages--; pfn++) {
 		struct page *page = pfn_to_page(pfn);
 
-		__ClearPageOffline(page);
 		if (!onlined)
+			/* generic_online_page() will clear PageOffline(). */
 			ClearPageDirty(page);
+		else
+			__ClearPageOffline(page);
 	}
 }
 
diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index aaf2514fcfa4..528395133b4f 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -146,7 +146,8 @@ static DECLARE_WAIT_QUEUE_HEAD(balloon_wq);
 /* balloon_append: add the given page to the balloon. */
 static void balloon_append(struct page *page)
 {
-	__SetPageOffline(page);
+	if (!PageOffline(page))
+		__SetPageOffline(page);
 
 	/* Lowmem is re-populated first, so highmem pages go at list tail. */
 	if (PageHighMem(page)) {
@@ -412,7 +413,11 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
 
 		xenmem_reservation_va_mapping_update(1, &page, &frame_list[i]);
 
-		/* Relinquish the page back to the allocator. */
+		/*
+		 * Relinquish the page back to the allocator. Note that
+		 * some pages, including ones added via xen_online_page(), might
+		 * not be marked reserved; free_reserved_page() will handle that.
+		 */
 		free_reserved_page(page);
 	}