mm: move hugetlb specific things in folio to page[3]

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.

Link: https://lkml.kernel.org/r/20250303163014.1128035-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: davidhildenbrand

include/linux/mm_types.h

@@ -407,20 +407,23 @@ 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;
 	};
 };
 
@@ -457,8 +460,12 @@ 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
 
 /**

mm/hugetlb.c

@@ -1649,7 +1649,6 @@ 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);
 }
 

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");