[PATCH v2 2/2] mm: wrap __find_buddy_pfn() with a necessary buddy page validation.

[Zi Yan]

From: Zi Yan <ziy@xxxxxxxxxx>

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.

Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/linux-mm/CAHk-=wji_AmYygZMTsPMdJ7XksMt7kOur8oDfDdniBRMjm4VkQ@xxxxxxxxxxxxxx/
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
 mm/internal.h       | 23 +----------------
 mm/page_alloc.c     | 63 ++++++++++++++++++++++++++++++++++++++-------
 mm/page_isolation.c |  8 ++----
 3 files changed, 56 insertions(+), 38 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 876e66237c89..f43565ed5ff6 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -211,28 +211,7 @@ struct alloc_context {
 	bool spread_dirty_pages;
 };
 
-/*
- * Locate the struct page for both the matching buddy in our
- * pair (buddy1) and the combined O(n+1) page they form (page).
- *
- * 1) Any buddy B1 will have an order O twin B2 which satisfies
- * the following equation:
- *     B2 = B1 ^ (1 << O)
- * For example, if the starting buddy (buddy2) is #8 its order
- * 1 buddy is #10:
- *     B2 = 8 ^ (1 << 1) = 8 ^ 2 = 10
- *
- * 2) Any buddy B will have an order O+1 parent P which
- * satisfies the following equation:
- *     P = B & ~(1 << O)
- *
- * Assumption: *_mem_map is contiguous at least up to MAX_ORDER
- */
-static inline unsigned long
-__find_buddy_pfn(unsigned long page_pfn, unsigned int order)
-{
-	return page_pfn ^ (1 << order);
-}
+extern struct page *find_buddy_page_pfn(struct page *page, unsigned int order);
 
 extern struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
 				unsigned long end_pfn, struct zone *zone);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ea106146686..8aedc6fbfdd0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -998,6 +998,51 @@ static inline void del_page_from_free_list(struct page *page, struct zone *zone,
 	zone->free_area[order].nr_free--;
 }
 
+/*
+ * Locate the struct page for both the matching buddy in our
+ * pair (buddy1) and the combined O(n+1) page they form (page).
+ *
+ * 1) Any buddy B1 will have an order O twin B2 which satisfies
+ * the following equation:
+ *     B2 = B1 ^ (1 << O)
+ * For example, if the starting buddy (buddy2) is #8 its order
+ * 1 buddy is #10:
+ *     B2 = 8 ^ (1 << 1) = 8 ^ 2 = 10
+ *
+ * 2) Any buddy B will have an order O+1 parent P which
+ * satisfies the following equation:
+ *     P = B & ~(1 << O)
+ *
+ * Assumption: *_mem_map is contiguous at least up to MAX_ORDER
+ */
+static inline unsigned long
+__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
+ * @order: The order of the input page
+ *
+ * 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.
+ */
+struct page *find_buddy_page_pfn(struct page *page, unsigned int order)
+{
+	unsigned long pfn = page_to_pfn(page);
+	unsigned long buddy_pfn = __find_buddy_pfn(pfn, order);
+	struct page *buddy = page + (buddy_pfn - pfn);
+
+	if (page_is_buddy(page, buddy, order))
+		return buddy;
+	return NULL;
+}
+
 /*
  * If this is not the largest possible page, check if the buddy
  * of the next-highest order is free. If it is, it's possible
@@ -1010,18 +1055,16 @@ static inline bool
 buddy_merge_likely(unsigned long pfn, unsigned long buddy_pfn,
 		   struct page *page, unsigned int order)
 {
-	struct page *higher_page, *higher_buddy;
-	unsigned long combined_pfn;
+	struct page *higher_page;
+	unsigned long higher_page_pfn;
 
 	if (order >= MAX_ORDER - 2)
 		return false;
 
-	combined_pfn = buddy_pfn & pfn;
-	higher_page = page + (combined_pfn - pfn);
-	buddy_pfn = __find_buddy_pfn(combined_pfn, order + 1);
-	higher_buddy = higher_page + (buddy_pfn - combined_pfn);
+	higher_page_pfn = buddy_pfn & pfn;
+	higher_page = page + (higher_page_pfn - pfn);
 
-	return page_is_buddy(higher_page, higher_buddy, order + 1);
+	return find_buddy_page_pfn(higher_page, order + 1) != NULL;
 }
 
 /*
@@ -1075,11 +1118,11 @@ static inline void __free_one_page(struct page *page,
 								migratetype);
 			return;
 		}
-		buddy_pfn = __find_buddy_pfn(pfn, order);
-		buddy = page + (buddy_pfn - pfn);
 
-		if (!page_is_buddy(page, buddy, order))
+		buddy = find_buddy_page_pfn(page, order);
+		if (!buddy)
 			goto done_merging;
+		buddy_pfn = page_to_pfn(buddy);
 
 		if (unlikely(order >= pageblock_order)) {
 			/*
diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index f67c4c70f17f..7326c9f57d55 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -70,7 +70,6 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	unsigned long flags, nr_pages;
 	bool isolated_page = false;
 	unsigned int order;
-	unsigned long pfn, buddy_pfn;
 	struct page *buddy;
 
 	zone = page_zone(page);
@@ -89,11 +88,8 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
 	if (PageBuddy(page)) {
 		order = buddy_order(page);
 		if (order >= pageblock_order && order < MAX_ORDER - 1) {
-			pfn = page_to_pfn(page);
-			buddy_pfn = __find_buddy_pfn(pfn, order);
-			buddy = page + (buddy_pfn - pfn);
-
-			if (!is_migrate_isolate_page(buddy)) {
+			buddy = find_buddy_page_pfn(page, order);
+			if (buddy && !is_migrate_isolate_page(buddy)) {
 				isolated_page = !!__isolate_free_page(page, order);
 				/*
 				 * Isolating a free page in an isolated pageblock
-- 
2.35.1