[PATCH v2 2/2] tools/tests/native: Add test for offlined buddy head PFN_ORDER

[Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>]

Add a regression test to check that the PFN_ORDER(pg) of buddy heads
is reset to 0 when they become offline single pages which have been
moved of the offlined page lists.

Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
---
 tools/tests/native/offline-head-order.c | 74 +++++++++++++++++++++++++
 1 file changed, 74 insertions(+)
 create mode 100644 tools/tests/native/offline-head-order.c

diff --git a/tools/tests/native/offline-head-order.c 
b/tools/tests/native/offline-head-order.c
new file mode 100644
index 000000000000..20c4f36526c1
--- /dev/null
+++ b/tools/tests/native/offline-head-order.c
@@ -0,0 +1,74 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Tests using offline_page() to verify reserve_offlined_page()
+ *
+ * The workflow tested here is offlining a free page:
+ *
+ * 1. offline_page() calls mark_page_offlined() to mark the page.
+ * 2. It calls reserve_heap_page() to find the containing buddy.
+ * 3. It calls reserve_offlined_page() to reserve the marked pages within
+ *    that buddy.
+ *
+ * reserve_offlined_page() then:
+ *
+ * 1. Removes the buddy, a 2^order group of pages, from the free list.
+ * 2. Finds size-aligned spans of healthy pages within it.
+ * 3. Rebuilds healthy buddies from those spans and
+ *    adds them back to the free list via page_list_add_scrub().
+ * 4. Moves offlined subpages to the offlined page lists.
+ *
+ * Copyright (C) 2026 Cloud Software Group
+ */
+#define CONFIG_SYSCTL
+#include "harness/native.h"
+
+static void test_offline_head_order(int start_mfn)
+{
+    struct page_info *page = frame_table + start_mfn;
+    uint32_t status = 0;
+
+    /* Seed a single order-1 buddy onto the heap. */
+    test_page_list_add_buddy(page, order1);
+    ASSERT(PFN_ORDER(page) == 1);
+    /* Offline the head page. */
+    ASSERT(offline_page(page_to_mfn(page), 0, &status) == 0);
+    ASSERT(status == PG_OFFLINE_OFFLINED);
+
+    /* Check the order of the offlined head page. */
+    ASSERT(PFN_ORDER(page) == 0);
+
+    /*
+     * Allocate the successor page of the offlined page. This prevents
+     * the normal successor page merge when the page is re-onlined below.
+     */
+    struct page_info *pg = alloc_domheap_pages(dom1, order0, 0);
+    ASSERT(pg == page + 1);
+    ASSERT(FREE_PAGES == 0);
+
+    /* Online the offlined former head page. */
+    ASSERT(online_page(page_to_mfn(page), &status) == 0);
+    ASSERT(status & PG_ONLINE_ONLINED);
+    ASSERT(FREE_PAGES == 1);
+
+    /*
+     * Confirm the order of the onlined former head page is 0, independently
+     * of the order returned by PFN_ORDER() for the offlined page. This should
+     * always be successful because page_offlined_list only contains single
+     * pages and online_page() ignores PFN_ORDER(pg) of the page. It calls
+     * free_heap_pages() passing the order hardcoded to 0. This causes it to
+     * pass the given order 0 to page_list_add_scrub(). This causes it to set
+     * the order of the page to 0 before it adds the page to the free list.
+     */
+    ASSERT(PFN_ORDER(page) == 0);
+}
+
+int main(int argc, char *argv[])
+{
+    const char *topic = "Test offlined head page to be updated to PFN_ORDER 0";
+    if ( !parse_args(argc, argv, topic) )
+        return EXIT_FAILURE;
+
+    init_page_alloc_tests();
+    RUN_TESTCASE("TOHP", test_offline_head_order, 2);
+    return test_complete();
+}
-- 
2.39.5