[PATCH v3 05/20] mm: move _pincount 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].

For order-1 folios (never anon folios!) on 32bit, we will now also use the
GUP_PIN_COUNTING_BIAS approach. A fully-mapped order-1 folio requires
2 references. With GUP_PIN_COUNTING_BIAS being 1024, we'd detect such
folios as "maybe pinned" with 512 full mappings, instead of 1024 for
order-0. As anon folios are out of the picture (which are the most relevant
users of checking for pinnings on *mapped* pages) and we are talking about
32bit, this is not expected to cause any trouble.

In __dump_page(), copy one additional folio page if we detect a folio
with an order > 1, so we can dump the pincount on order > 1 folios
reliably.

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", fortunately the 32bit
specifics will likely go away again; even small folios could then have a
pincount and folio_has_pincount() would essentially always return
"true".

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/mm.h       | 11 +++++++++--
 include/linux/mm_types.h |  5 +++++
 mm/debug.c               | 10 +++++++++-
 mm/gup.c                 |  8 ++++----
 mm/internal.h            |  3 ++-
 mm/page_alloc.c          | 14 +++++++++++---
 6 files changed, 40 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 694704217df8a..c1414491c0de2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2004,6 +2004,13 @@ static inline struct folio *pfn_folio(unsigned long pfn)
 	return page_folio(pfn_to_page(pfn));
 }
 
+static inline bool folio_has_pincount(const struct folio *folio)
+{
+	if (IS_ENABLED(CONFIG_64BIT))
+		return folio_test_large(folio);
+	return folio_order(folio) > 1;
+}
+
 /**
  * folio_maybe_dma_pinned - Report if a folio may be pinned for DMA.
  * @folio: The folio.
@@ -2020,7 +2027,7 @@ static inline struct folio *pfn_folio(unsigned long pfn)
  * get that many refcounts, and b) all the callers of this routine are
  * expected to be able to deal gracefully with a false positive.
  *
- * For large folios, the result will be exactly correct. That's because
+ * For most large folios, the result will be exactly correct. That's because
  * we have more tracking data available: the _pincount field is used
  * instead of the GUP_PIN_COUNTING_BIAS scheme.
  *
@@ -2031,7 +2038,7 @@ static inline struct folio *pfn_folio(unsigned long pfn)
  */
 static inline bool folio_maybe_dma_pinned(struct folio *folio)
 {
-	if (folio_test_large(folio))
+	if (folio_has_pincount(folio))
 		return atomic_read(&folio->_pincount) > 0;
 
 	/*
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 1d9c68c551d42..31f466d8485bc 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -387,7 +387,9 @@ struct folio {
 					atomic_t _large_mapcount;
 					atomic_t _entire_mapcount;
 					atomic_t _nr_pages_mapped;
+#ifdef CONFIG_64BIT
 					atomic_t _pincount;
+#endif /* CONFIG_64BIT */
 				};
 				unsigned long _usable_1[4];
 			};
@@ -406,6 +408,9 @@ struct folio {
 			unsigned long _head_2;
 	/* public: */
 			struct list_head _deferred_list;
+#ifndef CONFIG_64BIT
+			atomic_t _pincount;
+#endif /* !CONFIG_64BIT */
 	/* private: the union with struct page is transitional */
 		};
 		struct page __page_2;
diff --git a/mm/debug.c b/mm/debug.c
index 2d1bd67d957bc..83ef3bd0ccd32 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -79,12 +79,17 @@ static void __dump_folio(struct folio *folio, struct page *page,
 			folio_ref_count(folio), mapcount, mapping,
 			folio->index + idx, pfn);
 	if (folio_test_large(folio)) {
+		int pincount = 0;
+
+		if (folio_has_pincount(folio))
+			pincount = atomic_read(&folio->_pincount);
+
 		pr_warn("head: order:%u mapcount:%d entire_mapcount:%d nr_pages_mapped:%d pincount:%d\n",
 				folio_order(folio),
 				folio_mapcount(folio),
 				folio_entire_mapcount(folio),
 				folio_nr_pages_mapped(folio),
-				atomic_read(&folio->_pincount));
+				pincount);
 	}
 
 #ifdef CONFIG_MEMCG
@@ -146,6 +151,9 @@ static void __dump_page(const struct page *page)
 	if (idx < MAX_FOLIO_NR_PAGES) {
 		memcpy(&folio, foliop, 2 * sizeof(struct page));
 		nr_pages = folio_nr_pages(&folio);
+		if (nr_pages > 1)
+			memcpy(&folio.__page_2, &foliop->__page_2,
+			       sizeof(struct page));
 		foliop = &folio;
 	}
 
diff --git a/mm/gup.c b/mm/gup.c
index e5040657870ea..2944fe8cf3174 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -109,7 +109,7 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 		if (is_zero_folio(folio))
 			return;
 		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
-		if (folio_test_large(folio))
+		if (folio_has_pincount(folio))
 			atomic_sub(refs, &folio->_pincount);
 		else
 			refs *= GUP_PIN_COUNTING_BIAS;
@@ -164,7 +164,7 @@ int __must_check try_grab_folio(struct folio *folio, int refs,
 		 * Increment the normal page refcount field at least once,
 		 * so that the page really is pinned.
 		 */
-		if (folio_test_large(folio)) {
+		if (folio_has_pincount(folio)) {
 			folio_ref_add(folio, refs);
 			atomic_add(refs, &folio->_pincount);
 		} else {
@@ -223,7 +223,7 @@ void folio_add_pin(struct folio *folio)
 	 * page refcount field at least once, so that the page really is
 	 * pinned.
 	 */
-	if (folio_test_large(folio)) {
+	if (folio_has_pincount(folio)) {
 		WARN_ON_ONCE(atomic_read(&folio->_pincount) < 1);
 		folio_ref_inc(folio);
 		atomic_inc(&folio->_pincount);
@@ -575,7 +575,7 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
 	 * is pinned.  That's why the refcount from the earlier
 	 * try_get_folio() is left intact.
 	 */
-	if (folio_test_large(folio))
+	if (folio_has_pincount(folio))
 		atomic_add(refs, &folio->_pincount);
 	else
 		folio_ref_add(folio,
diff --git a/mm/internal.h b/mm/internal.h
index bcda1f604038f..378464246f259 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -721,7 +721,8 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
 	atomic_set(&folio->_large_mapcount, -1);
 	atomic_set(&folio->_entire_mapcount, -1);
 	atomic_set(&folio->_nr_pages_mapped, 0);
-	atomic_set(&folio->_pincount, 0);
+	if (IS_ENABLED(CONFIG_64BIT) || order > 1)
+		atomic_set(&folio->_pincount, 0);
 	if (order > 1)
 		INIT_LIST_HEAD(&folio->_deferred_list);
 }
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2fc03cb13e49d..594a552c735cd 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -963,9 +963,11 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 			bad_page(page, "nonzero nr_pages_mapped");
 			goto out;
 		}
-		if (unlikely(atomic_read(&folio->_pincount))) {
-			bad_page(page, "nonzero pincount");
-			goto out;
+		if (IS_ENABLED(CONFIG_64BIT)) {
+			if (unlikely(atomic_read(&folio->_pincount))) {
+				bad_page(page, "nonzero pincount");
+				goto out;
+			}
 		}
 		break;
 	case 2:
@@ -974,6 +976,12 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
 			bad_page(page, "on deferred list");
 			goto out;
 		}
+		if (!IS_ENABLED(CONFIG_64BIT)) {
+			if (unlikely(atomic_read(&folio->_pincount))) {
+				bad_page(page, "nonzero pincount");
+				goto out;
+			}
+		}
 		break;
 	case 3:
 		/* the third tail page: hugetlb specifics overlap ->mappings */
-- 
2.48.1