mm/rmap: use folio->_mapcount for small folios

We have some cases left whereby we operate on small folios and still refer
to page->_mapcount.  Let's just use folio->_mapcount instead, which
currently still overlays page->_mapcount, so no change.

This change will make it easier to later spot any remaining users of
page->_mapcount that target tail pages.

Link: https://lkml.kernel.org/r/20240816103246.719209-1-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/rmap.h

@@ -331,7 +331,7 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
 	switch (level) {
 	case RMAP_LEVEL_PTE:
 		if (!folio_test_large(folio)) {
-			atomic_inc(&page->_mapcount);
+			atomic_inc(&folio->_mapcount);
 			break;
 		}
 
@@ -425,7 +425,7 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
 		if (!folio_test_large(folio)) {
 			if (PageAnonExclusive(page))
 				ClearPageAnonExclusive(page);
-			atomic_inc(&page->_mapcount);
+			atomic_inc(&folio->_mapcount);
 			break;
 		}
 

mm/rmap.c

@@ -1165,7 +1165,7 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
 	switch (level) {
 	case RMAP_LEVEL_PTE:
 		if (!folio_test_large(folio)) {
-			nr = atomic_inc_and_test(&page->_mapcount);
+			nr = atomic_inc_and_test(&folio->_mapcount);
 			break;
 		}
 
@@ -1535,7 +1535,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
 	switch (level) {
 	case RMAP_LEVEL_PTE:
 		if (!folio_test_large(folio)) {
-			nr = atomic_add_negative(-1, &page->_mapcount);
+			nr = atomic_add_negative(-1, &folio->_mapcount);
 			break;
 		}