mm: Turn can_split_huge_page() into can_split_folio()

This function already required a head page to be passed, so this
just adds type-safety and removes a few implicit calls to
compound_head().
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

include/linux/huge_mm.h

@@ -185,7 +185,7 @@ void prep_transhuge_page(struct page *page);
 void free_transhuge_page(struct page *page);
 bool is_transparent_hugepage(struct page *page);
 
-bool can_split_huge_page(struct page *page, int *pextra_pins);
+bool can_split_folio(struct folio *folio, int *pextra_pins);
 int split_huge_page_to_list(struct page *page, struct list_head *list);
 static inline int split_huge_page(struct page *page)
 {
@@ -387,7 +387,7 @@ static inline bool is_transparent_hugepage(struct page *page)
 #define thp_get_unmapped_area	NULL
 
 static inline bool
-can_split_huge_page(struct page *page, int *pextra_pins)
+can_split_folio(struct folio *folio, int *pextra_pins)
 {
 	BUILD_BUG();
 	return false;

mm/huge_memory.c

@@ -2516,18 +2516,19 @@ int page_trans_huge_mapcount(struct page *page)
 }
 
 /* Racy check whether the huge page can be split */
-bool can_split_huge_page(struct page *page, int *pextra_pins)
+bool can_split_folio(struct folio *folio, int *pextra_pins)
 {
 	int extra_pins;
 
 	/* Additional pins from page cache */
-	if (PageAnon(page))
-		extra_pins = PageSwapCache(page) ? thp_nr_pages(page) : 0;
+	if (folio_test_anon(folio))
+		extra_pins = folio_test_swapcache(folio) ?
+				folio_nr_pages(folio) : 0;
 	else
-		extra_pins = thp_nr_pages(page);
+		extra_pins = folio_nr_pages(folio);
 	if (pextra_pins)
 		*pextra_pins = extra_pins;
-	return total_mapcount(page) == page_count(page) - extra_pins - 1;
+	return folio_mapcount(folio) == folio_ref_count(folio) - extra_pins - 1;
 }
 
 /*
@@ -2619,7 +2620,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
 	 * Racy check if we can split the page, before unmap_page() will
 	 * split PMDs
 	 */
-	if (!can_split_huge_page(head, &extra_pins)) {
+	if (!can_split_folio(folio, &extra_pins)) {
 		ret = -EBUSY;
 		goto out_unlock;
 	}
@@ -2928,7 +2929,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 			goto next;
 
 		total++;
-		if (!can_split_huge_page(compound_head(page), NULL))
+		if (!can_split_folio(page_folio(page), NULL))
 			goto next;
 
 		if (!trylock_page(page))

mm/vmscan.c

@@ -1703,18 +1703,18 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 			if (!PageSwapCache(page)) {
 				if (!(sc->gfp_mask & __GFP_IO))
 					goto keep_locked;
-				if (page_maybe_dma_pinned(page))
+				if (folio_maybe_dma_pinned(folio))
 					goto keep_locked;
 				if (PageTransHuge(page)) {
 					/* cannot split THP, skip it */
-					if (!can_split_huge_page(page, NULL))
+					if (!can_split_folio(folio, NULL))
 						goto activate_locked;
 					/*
 					 * Split pages without a PMD map right
 					 * away. Chances are some or all of the
 					 * tail pages can be freed without IO.
 					 */
-					if (!compound_mapcount(page) &&
+					if (!folio_entire_mapcount(folio) &&
 					    split_folio_to_list(folio,
 								page_list))
 						goto activate_locked;