[PATCH v2 1/2] xen/mm: reset PFN_ORDER for offlined buddy heads

[Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>]

Ensure offlined buddy head pages are annotated as order-0 pages.

When a buddy containing pages marked for offlining is processed,
reserve_offlined_page() rebuilds any surviving healthy buddies
and moves the offlined subpages onto the offlined lists.

If the buddy head itself is offlined it was previously left
annotated with the original buddy order even though it has
been split into a single page.

This has no functional impact as the order of an offlined
page is not used for any decision making and onlining, but
it is misleading when inspecting the page's metadata.

Set PFN_ORDER(cur_head) to 0 for all pages moved to the
lists of offlined and broken pages so the page's stored
order reflects its actual size.

Signed-off-by: Bernhard Kaindl <bernhard.kaindl@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2:
- Fix isolated from the new test and moved it ahead of the test.
- Unconditionally store 0 as the PFN_ORDER() of the offlined
  pages to make the code more robust against future changes.
---
 xen/common/page_alloc.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index e01ac3e99c72..6c4e26004b2a 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1251,6 +1251,12 @@ static int reserve_offlined_page(struct page_info *head)
         ASSERT(total_avail_pages > 0);
         total_avail_pages--;
 
+        /*
+         * All offlined pages are standalone pages: If this offlined page was
+         * the head of a higher-order buddy, we need to reset its order to 0:
+         */
+        PFN_ORDER(cur_head) = 0;
+
         page_list_add_tail(cur_head,
                            test_bit(_PGC_broken, &cur_head->count_info) ?
                            &page_broken_list : &page_offlined_list);
-- 
2.39.5