[PATCH v2 04/20] mm: move hugetlb specific things in folio to page[3]

[David Hildenbrand <david@xxxxxxxxxx>]

Let's just move the hugetlb specific stuff to a separate page, and stop
letting it overlay other fields for now.

This frees up some space in page[2], which we will use on 32bit to free
up some space in page[1]. While we could move these things to page[3]
instead, it's cleaner to just move the hugetlb specific things out of
the way and pack the core-folio stuff as tight as possible. ... and we
can minimize the work required in dump_folio.

We can now avoid re-initializing &folio->_deferred_list in hugetlb code.

Hopefully dynamically allocating "strut folio" in the future will further
clean this up.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm_types.h | 27 +++++++++++++++++----------
 mm/hugetlb.c             |  1 -
 mm/page_alloc.c          |  5 +++++
 3 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index e81be20bbabc6..1d9c68c551d42 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -405,20 +405,23 @@ struct folio {
 			unsigned long _flags_2;
 			unsigned long _head_2;
 	/* public: */
-			void *_hugetlb_subpool;
-			void *_hugetlb_cgroup;
-			void *_hugetlb_cgroup_rsvd;
-			void *_hugetlb_hwpoison;
+			struct list_head _deferred_list;
 	/* private: the union with struct page is transitional */
 		};
+		struct page __page_2;
+	};
+	union {
 		struct {
-			unsigned long _flags_2a;
-			unsigned long _head_2a;
+			unsigned long _flags_3;
+			unsigned long _head_3;
 	/* public: */
-			struct list_head _deferred_list;
+			void *_hugetlb_subpool;
+			void *_hugetlb_cgroup;
+			void *_hugetlb_cgroup_rsvd;
+			void *_hugetlb_hwpoison;
 	/* private: the union with struct page is transitional */
 		};
-		struct page __page_2;
+		struct page __page_3;
 	};
 };
 
@@ -455,8 +458,12 @@ FOLIO_MATCH(_refcount, _refcount_1);
 			offsetof(struct page, pg) + 2 * sizeof(struct page))
 FOLIO_MATCH(flags, _flags_2);
 FOLIO_MATCH(compound_head, _head_2);
-FOLIO_MATCH(flags, _flags_2a);
-FOLIO_MATCH(compound_head, _head_2a);
+#undef FOLIO_MATCH
+#define FOLIO_MATCH(pg, fl)						\
+	static_assert(offsetof(struct folio, fl) ==			\
+			offsetof(struct page, pg) + 3 * sizeof(struct page))
+FOLIO_MATCH(flags, _flags_3);
+FOLIO_MATCH(compound_head, _head_3);
 #undef FOLIO_MATCH
 
 /**
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9faa1034704ff..2ad5c292568ab 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1646,7 +1646,6 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
 
 	folio_ref_unfreeze(folio, 1);
 
-	INIT_LIST_HEAD(&folio->_deferred_list);
 	hugetlb_free_folio(folio);
 }
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bdfc954dab9aa..05a2a9492cdb0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -971,6 +971,11 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 			goto out;
 		}
 		break;
+	case 3:
+		/* the third tail page: hugetlb specifics overlap ->mappings */
+		if (IS_ENABLED(CONFIG_HUGETLB_PAGE))
+			break;
+		fallthrough;
 	default:
 		if (page->mapping != TAIL_MAPPING) {
 			bad_page(page, "corrupted mapping in tail page");
-- 
2.48.1