[PATCH v2 06/20] mm: move _entire_mapcount in folio to page[2] on 32bit

[David Hildenbrand <david@redhat.com>]

Let's free up some space on 32bit in page[1] by moving the _pincount to
page[2].

Ordinary folios only use the entire mapcount with PMD mappings, so
order-1 folios don't apply. Similarly, hugetlb folios are always larger
than order-1, turning the entire mapcount essentially unused for all
order-1 folios. Moving it to order-1 folios will not change anything.

On 32bit, simply check in folio_entire_mapcount() whether we have an
order-1 folio, and return 0 in that case.

Note that THPs on 32bit are not particularly common (and we don't care
too much about performance), but we want to keep it working reliably,
because likely we want to use large folios there as well in the future,
independent of PMD leaf support.

Once we dynamically allocate "struct folio", the 32bit specifics will go
away again; even small folios could then have a pincount.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/mm.h       |  2 ++
 include/linux/mm_types.h |  3 ++-
 mm/internal.h            |  5 +++--
 mm/page_alloc.c          | 12 ++++++++----
 4 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1a4ee028a851e..9c1290588a11e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1333,6 +1333,8 @@ static inline int is_vmalloc_or_module_addr(const void *x)
 static inline int folio_entire_mapcount(const struct folio *folio)
 {
 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+	if (!IS_ENABLED(CONFIG_64BIT) && unlikely(folio_large_order(folio) == 1))
+		return 0;
 	return atomic_read(&folio->_entire_mapcount) + 1;
 }
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 31f466d8485bc..c83dd2f1ee25e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -385,9 +385,9 @@ struct folio {
 			union {
 				struct {
 					atomic_t _large_mapcount;
-					atomic_t _entire_mapcount;
 					atomic_t _nr_pages_mapped;
 #ifdef CONFIG_64BIT
+					atomic_t _entire_mapcount;
 					atomic_t _pincount;
 #endif /* CONFIG_64BIT */
 				};
@@ -409,6 +409,7 @@ struct folio {
 	/* public: */
 			struct list_head _deferred_list;
 #ifndef CONFIG_64BIT
+			atomic_t _entire_mapcount;
 			atomic_t _pincount;
 #endif /* !CONFIG_64BIT */
 	/* private: the union with struct page is transitional */
diff --git a/mm/internal.h b/mm/internal.h
index d33db24c8b17b..ffdc91b19322e 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -721,10 +721,11 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 
 	folio_set_order(folio, order);
 	atomic_set(&folio->_large_mapcount, -1);
-	atomic_set(&folio->_entire_mapcount, -1);
 	atomic_set(&folio->_nr_pages_mapped, 0);
-	if (IS_ENABLED(CONFIG_64BIT) || order > 1)
+	if (IS_ENABLED(CONFIG_64BIT) || order > 1) {
 		atomic_set(&folio->_pincount, 0);
+		atomic_set(&folio->_entire_mapcount, -1);
+	}
 	if (order > 1)
 		INIT_LIST_HEAD(&folio->_deferred_list);
 }
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3dff99cc54161..7036530bd1bca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -947,10 +947,6 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 	switch (page - head_page) {
 	case 1:
 		/* the first tail page: these may be in place of ->mapping */
-		if (unlikely(folio_entire_mapcount(folio))) {
-			bad_page(page, "nonzero entire_mapcount");
-			goto out;
-		}
 		if (unlikely(folio_large_mapcount(folio))) {
 			bad_page(page, "nonzero large_mapcount");
 			goto out;
@@ -960,6 +956,10 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 			goto out;
 		}
 		if (IS_ENABLED(CONFIG_64BIT)) {
+			if (unlikely(atomic_read(&folio->_entire_mapcount) + 1)) {
+				bad_page(page, "nonzero entire_mapcount");
+				goto out;
+			}
 			if (unlikely(atomic_read(&folio->_pincount))) {
 				bad_page(page, "nonzero pincount");
 				goto out;
@@ -973,6 +973,10 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 			goto out;
 		}
 		if (!IS_ENABLED(CONFIG_64BIT)) {
+			if (unlikely(atomic_read(&folio->_entire_mapcount) + 1)) {
+				bad_page(page, "nonzero entire_mapcount");
+				goto out;
+			}
 			if (unlikely(atomic_read(&folio->_pincount))) {
 				bad_page(page, "nonzero pincount");
 				goto out;
-- 
2.48.1