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>

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 */

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
@@ -366,6 +361,13 @@ struct folio {
 			void *_hugetlb_cgroup;
 			void *_hugetlb_cgroup_rsvd;
 			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;

mm/huge_memory.c

@@ -2756,9 +2756,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	/* Prevent deferred_split_scan() touching ->_refcount */
 	spin_lock(&ds_queue->split_queue_lock);
 	if (folio_ref_freeze(folio, 1 + extra_pins)) {
-		if (!list_empty(page_deferred_list(&folio->page))) {
+		if (!list_empty(&folio->_deferred_list)) {
 			ds_queue->split_queue_len--;
-			list_del(page_deferred_list(&folio->page));
+			list_del(&folio->_deferred_list);
 		}
 		spin_unlock(&ds_queue->split_queue_lock);
 		if (mapping) {
@@ -2873,8 +2873,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 	struct pglist_data *pgdata = NODE_DATA(sc->nid);
 	struct deferred_split *ds_queue = &pgdata->deferred_split_queue;
 	unsigned long flags;
-	LIST_HEAD(list), *pos, *next;
-	struct page *page;
+	LIST_HEAD(list);
+	struct folio *folio, *next;
 	int split = 0;
 
 #ifdef CONFIG_MEMCG
@@ -2884,14 +2884,13 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 
 	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
 	/* Take pin on all head pages to avoid freeing them under us */
-	list_for_each_safe(pos, next, &ds_queue->split_queue) {
-		page = list_entry((void *)pos, struct page, deferred_list);
-		page = compound_head(page);
-		if (get_page_unless_zero(page)) {
-			list_move(page_deferred_list(page), &list);
+	list_for_each_entry_safe(folio, next, &ds_queue->split_queue,
+							_deferred_list) {
+		if (folio_try_get(folio)) {
+			list_move(&folio->_deferred_list, &list);
 		} else {
-			/* We lost race with put_compound_page() */
-			list_del_init(page_deferred_list(page));
+			/* We lost race with folio_put() */
+			list_del_init(&folio->_deferred_list);
 			ds_queue->split_queue_len--;
 		}
 		if (!--sc->nr_to_scan)
@@ -2899,16 +2898,15 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 	}
 	spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
 
-	list_for_each_safe(pos, next, &list) {
-		page = list_entry((void *)pos, struct page, deferred_list);
-		if (!trylock_page(page))
+	list_for_each_entry_safe(folio, next, &list, _deferred_list) {
+		if (!folio_trylock(folio))
 			goto next;
 		/* split_huge_page() removes page from list on success */
-		if (!split_huge_page(page))
+		if (!split_folio(folio))
 			split++;
-		unlock_page(page);
+		folio_unlock(folio);
 next:
-		put_page(page);
+		folio_put(folio);
 	}
 
 	spin_lock_irqsave(&ds_queue->split_queue_lock, flags);