mm: wrap __find_buddy_pfn() with a necessary buddy page validation

Whenever the buddy of a page is found from __find_buddy_pfn(),
page_is_buddy() should be used to check its validity.  Add a helper
function find_buddy_page_pfn() to find the buddy page and do the check
together.

[ziy@nvidia.com: updates per David]
Link: https://lkml.kernel.org/r/20220401230804.1658207-2-zi.yan@sent.com
Link: https://lore.kernel.org/linux-mm/CAHk-=wji_AmYygZMTsPMdJ7XksMt7kOur8oDfDdniBRMjm4VkQ@mail.gmail.com/
Link: https://lkml.kernel.org/r/7236E7CA-B5F1-4C04-AB85-E86FA3E9A54B@nvidia.com
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 90 insertions, 27 deletions

diff --git a/mm/internal.h b/mm/internal.h
index cf16280ce132..6293468fd7b7 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -212,6 +212,67 @@ struct alloc_context {
 };
 
 /*
+ * This function returns the order of a free page in the buddy system. In
+ * general, page_zone(page)->lock must be held by the caller to prevent the
+ * page from being allocated in parallel and returning garbage as the order.
+ * If a caller does not hold page_zone(page)->lock, it must guarantee that the
+ * page cannot be allocated or merged in parallel. Alternatively, it must
+ * handle invalid values gracefully, and use buddy_order_unsafe() below.
+ */
+static inline unsigned int buddy_order(struct page *page)
+{
+	/* PageBuddy() must be checked by the caller */
+	return page_private(page);
+}
+
+/*
+ * Like buddy_order(), but for callers who cannot afford to hold the zone lock.
+ * PageBuddy() should be checked first by the caller to minimize race window,
+ * and invalid values must be handled gracefully.
+ *
+ * READ_ONCE is used so that if the caller assigns the result into a local
+ * variable and e.g. tests it for valid range before using, the compiler cannot
+ * decide to remove the variable and inline the page_private(page) multiple
+ * times, potentially observing different values in the tests and the actual
+ * use of the result.
+ */
+#define buddy_order_unsafe(page)	READ_ONCE(page_private(page))
+
+/*
+ * This function checks whether a page is free && is the buddy
+ * we can coalesce a page and its buddy if
+ * (a) the buddy is not in a hole (check before calling!) &&
+ * (b) the buddy is in the buddy system &&
+ * (c) a page and its buddy have the same order &&
+ * (d) a page and its buddy are in the same zone.
+ *
+ * For recording whether a page is in the buddy system, we set PageBuddy.
+ * Setting, clearing, and testing PageBuddy is serialized by zone->lock.
+ *
+ * For recording page's order, we use page_private(page).
+ */
+static inline bool page_is_buddy(struct page *page, struct page *buddy,
+				 unsigned int order)
+{
+	if (!page_is_guard(buddy) && !PageBuddy(buddy))
+		return false;
+
+	if (buddy_order(buddy) != order)
+		return false;
+
+	/*
+	 * zone check is done late to avoid uselessly calculating
+	 * zone/node ids for pages that could never merge.
+	 */
+	if (page_zone_id(page) != page_zone_id(buddy))
+		return false;
+
+	VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
+	return true;
+}
+
+/*
  * Locate the struct page for both the matching buddy in our
  * pair (buddy1) and the combined O(n+1) page they form (page).
  *
@@ -234,6 +295,35 @@ __find_buddy_pfn(unsigned long page_pfn, unsigned int order)
 	return page_pfn ^ (1 << order);
 }
 
+/*
+ * Find the buddy of @page and validate it.
+ * @page: The input page
+ * @pfn: The pfn of the page, it saves a call to page_to_pfn() when the
+ *       function is used in the performance-critical __free_one_page().
+ * @order: The order of the page
+ * @buddy_pfn: The output pointer to the buddy pfn, it also saves a call to
+ *             page_to_pfn().
+ *
+ * The found buddy can be a non PageBuddy, out of @page's zone, or its order is
+ * not the same as @page. The validation is necessary before use it.
+ *
+ * Return: the found buddy page or NULL if not found.
+ */
+static inline struct page *find_buddy_page_pfn(struct page *page,
+			unsigned long pfn, unsigned int order, unsigned long *buddy_pfn)
+{
+	unsigned long __buddy_pfn = __find_buddy_pfn(pfn, order);
+	struct page *buddy;
+
+	buddy = page + (__buddy_pfn - pfn);
+	if (buddy_pfn)
+		*buddy_pfn = __buddy_pfn;
+
+	if (page_is_buddy(page, buddy, order))
+		return buddy;
+	return NULL;
+}
+
 extern struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
 				unsigned long end_pfn, struct zone *zone);
 
@@ -337,33 +427,6 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
 			int migratetype, bool only_stealable, bool *can_steal);
 
 /*
- * This function returns the order of a free page in the buddy system. In
- * general, page_zone(page)->lock must be held by the caller to prevent the
- * page from being allocated in parallel and returning garbage as the order.
- * If a caller does not hold page_zone(page)->lock, it must guarantee that the
- * page cannot be allocated or merged in parallel. Alternatively, it must
- * handle invalid values gracefully, and use buddy_order_unsafe() below.
- */
-static inline unsigned int buddy_order(struct page *page)
-{
-	/* PageBuddy() must be checked by the caller */
-	return page_private(page);
-}
-
-/*
- * Like buddy_order(), but for callers who cannot afford to hold the zone lock.
- * PageBuddy() should be checked first by the caller to minimize race window,
- * and invalid values must be handled gracefully.
- *
- * READ_ONCE is used so that if the caller assigns the result into a local
- * variable and e.g. tests it for valid range before using, the compiler cannot
- * decide to remove the variable and inline the page_private(page) multiple
- * times, potentially observing different values in the tests and the actual
- * use of the result.
- */
-#define buddy_order_unsafe(page)	READ_ONCE(page_private(page))
-
-/*
  * These three helpers classifies VMAs for virtual memory accounting.
  */