mm: move page->deferred_list to folio->_deferred_list

Remove the entire block of definitions for the second tail page, and add
the deferred list to the struct folio.  This actually moves _deferred_list
to a different offset in struct folio because I don't see a need to
include the padding.

This lets us use list_for_each_entry_safe() in deferred_split_scan()
and avoid a number of calls to compound_head().

Link: https://lkml.kernel.org/r/20230111142915.1001531-25-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 12 insertions, 11 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index a1341fdcf666..aacfcb02606f 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -295,11 +295,10 @@ static inline bool thp_migration_supported(void)
 
 static inline struct list_head *page_deferred_list(struct page *page)
 {
-	/*
-	 * See organization of tail pages of compound page in
-	 * "struct page" definition.
-	 */
-	return &page[2].deferred_list;
+	struct folio *folio = (struct folio *)page;
+
+	VM_BUG_ON_FOLIO(folio_order(folio) < 2, folio);
+	return &folio->_deferred_list;
 }
 
 #else /* CONFIG_TRANSPARENT_HUGEPAGE */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d458e9b8496c..7eb4d0815a78 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -141,12 +141,6 @@ struct page {
 		struct {	/* Tail pages of compound page */
 			unsigned long compound_head;	/* Bit zero is set */
 		};
-		struct {	/* Second tail page of transparent huge page */
-			unsigned long _compound_pad_1;	/* compound_head */
-			unsigned long _compound_pad_2;
-			/* For both global and memcg */
-			struct list_head deferred_list;
-		};
 		struct {	/* Second tail page of hugetlb page */
 			unsigned long _hugetlb_pad_1;	/* compound_head */
 			void *hugetlb_subpool;
@@ -302,6 +296,7 @@ static inline struct page *encoded_page_ptr(struct encoded_page *page)
  * @_hugetlb_cgroup: Do not use directly, use accessor in hugetlb_cgroup.h.
  * @_hugetlb_cgroup_rsvd: Do not use directly, use accessor in hugetlb_cgroup.h.
  * @_hugetlb_hwpoison: Do not use directly, call raw_hwp_list_head().
+ * @_deferred_list: Folios to be split under memory pressure.
  *
  * A folio is a physically, virtually and logically contiguous set
  * of bytes.  It is a power-of-two in size, and it is aligned to that
@@ -368,6 +363,13 @@ struct folio {
 			void *_hugetlb_hwpoison;
 	/* private: the union with struct page is transitional */
 		};
+		struct {
+			unsigned long _flags_2a;
+			unsigned long _head_2a;
+	/* public: */
+			struct list_head _deferred_list;
+	/* private: the union with struct page is transitional */
+		};
 		struct page __page_2;
 	};
 };