[PATCH v2 2/2] tools/tests: Regression test checking unaligned pages after offlining

[Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>]

Add a regression test to check that offlining pages does not return
a misaligned buddy like to the free lists after reseving the first
offlined sub-page from a buddy and returning the healthy  to the free list:

   +---------------+-----------------+-----------------+----------------+
   | offlined page |     head page with a tail page    | single page    |
   +---------------+-----------------+-----------------+----------------+

After a seres of allocations and frees, such an unaligned buddy would
trigger a Xen BUG if this state is reached when the tail is allocated:

  +---------------+-----------------+-----------------+
  | offlined page |    head page         tail page    |
  +---------------+-----------------+-----------------+----------------+
                                    |   in-use page   |   in-use page  |
                                    +-----------------+----------------+

Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
---
 tools/tests/native/offline-unaligned.c | 60 ++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 tools/tests/native/offline-unaligned.c

diff --git a/tools/tests/native/offline-unaligned.c 
b/tools/tests/native/offline-unaligned.c
new file mode 100644
index 000000000000..198bf952ba7d
--- /dev/null
+++ b/tools/tests/native/offline-unaligned.c
@@ -0,0 +1,60 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Regression test offlining to not return misaligned buddies to free lists.
+ * Copyright (C) 2026 Cloud Software Group
+ */
+#include "harness/native.h"
+
+/*
+ * Verify that offlining the head page of an order-2 buddy does not return
+ * a misaligned buddy to the free list, and the pages returned to the free
+ * list are usable for allocations.
+ */
+static void test_unaligned_buddy_merge(int start_mfn)
+{
+    struct page_info *pg = frame_table + start_mfn;
+    uint32_t status = 0;
+
+    /*
+     * Prepare a valid order-2 buddy (4 pages) with this layout:
+     * +---------------+-----------------+-----------------+----------------+
+     * | head page     | tail page 1     | tail page 2     | tail page 3    |
+     * +---------------+-----------------+-----------------+----------------+
+     */
+    test_page_list_add_buddy(pg, order2);
+
+    /* Act */
+    offline_page(page_to_mfn(pg), 0, &status);
+    CHECK(status & PG_OFFLINE_OFFLINED, "Page should be offlined");
+
+    /*
+     * The expected free list state after offlining the buddy head is:
+     * +---------------+---------------+----------------+---------------+
+     * | offlined page |  single page  |    head page with a tail page  |
+     * +---------------+---------------+----------------+---------------+
+     */
+    CHECK(page_aligned(pg + 1), "The buddy #%lu is not aligned to order-%d",
+          mfn_x(page_to_mfn(pg + 1)), PFN_ORDER(pg + 1));
+
+    /* Allocate and free a page to trigger buddy merging on free. */
+    free_domheap_pages(alloc_domheap_pages(dom1, order0, 0), order0);
+    CHECK((pg = alloc_domheap_pages(dom1, order1, 0)), "Alloc the order-1 pg");
+
+    /* Inspect the predecessor (pg is the tail of the unaligned buddy) */
+    CHECK(page_aligned(pg - 1), "The buddy #%lu is not aligned to order-%d!",
+          mfn_x(page_to_mfn(pg - 1)), PFN_ORDER(pg - 1));
+
+    /* Test allocating the remaining page */
+    alloc_domheap_pages(dom1, order0, 0);
+}
+
+int main(int argc, char *argv[])
+{
+    if ( !parse_args(argc, argv, "Test offlining to return aligned buddies") )
+        return EXIT_FAILURE;
+
+    init_page_alloc_tests();
+    RUN_TESTCASE("TUBM", test_unaligned_buddy_merge, 4);
+
+    return test_complete();
+}
-- 
2.39.5