[PATCH 01/28] mm: Remove folio_pincount_ptr() and head_compound_pincount()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

We can use folio->_pincount directly, since all users are guarded by
tests of compound/large.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 Documentation/core-api/pin_user_pages.rst | 29 +++++++++++------------
 include/linux/mm.h                        | 14 ++---------
 include/linux/mm_types.h                  |  5 ----
 mm/debug.c                                |  4 ++--
 mm/gup.c                                  |  8 +++----
 mm/huge_memory.c                          |  4 ++--
 mm/hugetlb.c                              |  4 ++--
 mm/page_alloc.c                           |  9 ++++---
 8 files changed, 32 insertions(+), 45 deletions(-)

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index facafbdecb95..9fb0b1080d3b 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -55,18 +55,17 @@ flags the caller provides. The caller is required to pass in a non-null struct
 pages* array, and the function then pins pages by incrementing each by a special
 value: GUP_PIN_COUNTING_BIAS.
 
-For compound pages, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead,
-an exact form of pin counting is achieved, by using the 2nd struct page
-in the compound page. A new struct page field, compound_pincount, has
-been added in order to support this.
-
-This approach for compound pages avoids the counting upper limit problems that
-are discussed below. Those limitations would have been aggravated severely by
-huge pages, because each tail page adds a refcount to the head page. And in
-fact, testing revealed that, without a separate compound_pincount field,
-page overflows were seen in some huge page stress tests.
-
-This also means that huge pages and compound pages do not suffer
+For large folios, the GUP_PIN_COUNTING_BIAS scheme is not used. Instead,
+the extra space available in the struct folio is used to store the
+pincount directly.
+
+This approach for large folios avoids the counting upper limit problems
+that are discussed below. Those limitations would have been aggravated
+severely by huge pages, because each tail page adds a refcount to the
+head page. And in fact, testing revealed that, without a separate pincount
+field, refcount overflows were seen in some huge page stress tests.
+
+This also means that huge pages and large folios do not suffer
 from the false positives problem that is mentioned below.::
 
  Function
@@ -264,9 +263,9 @@ place.)
 Other diagnostics
 =================
 
-dump_page() has been enhanced slightly, to handle these new counting
-fields, and to better report on compound pages in general. Specifically,
-for compound pages, the exact (compound_pincount) pincount is reported.
+dump_page() has been enhanced slightly to handle these new counting
+fields, and to better report on large folios in general.  Specifically,
+for large folios, the exact pincount is reported.
 
 References
 ==========
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 49e40766adb6..5683a25ce08e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1011,11 +1011,6 @@ static inline void folio_set_compound_dtor(struct folio *folio,
 
 void destroy_large_folio(struct folio *folio);
 
-static inline int head_compound_pincount(struct page *head)
-{
-	return atomic_read(compound_pincount_ptr(head));
-}
-
 static inline void set_compound_order(struct page *page, unsigned int order)
 {
 	page[1].compound_order = order;
@@ -1641,11 +1636,6 @@ static inline struct folio *pfn_folio(unsigned long pfn)
 	return page_folio(pfn_to_page(pfn));
 }
 
-static inline atomic_t *folio_pincount_ptr(struct folio *folio)
-{
-	return &folio_page(folio, 1)->compound_pincount;
-}
-
 /**
  * folio_maybe_dma_pinned - Report if a folio may be pinned for DMA.
  * @folio: The folio.
@@ -1663,7 +1653,7 @@ static inline atomic_t *folio_pincount_ptr(struct folio *folio)
  * expected to be able to deal gracefully with a false positive.
  *
  * For large folios, the result will be exactly correct. That's because
- * we have more tracking data available: the compound_pincount is used
+ * we have more tracking data available: the _pincount field is used
  * instead of the GUP_PIN_COUNTING_BIAS scheme.
  *
  * For more information, please see Documentation/core-api/pin_user_pages.rst.
@@ -1674,7 +1664,7 @@ static inline atomic_t *folio_pincount_ptr(struct folio *folio)
 static inline bool folio_maybe_dma_pinned(struct folio *folio)
 {
 	if (folio_test_large(folio))
-		return atomic_read(folio_pincount_ptr(folio)) > 0;
+		return atomic_read(&folio->_pincount) > 0;
 
 	/*
 	 * folio_ref_count() is signed. If that refcount overflows, then
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 2b0a0595fc9e..c225d81eae83 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -443,11 +443,6 @@ static inline atomic_t *subpages_mapcount_ptr(struct page *page)
 	return &page[1].subpages_mapcount;
 }
 
-static inline atomic_t *compound_pincount_ptr(struct page *page)
-{
-	return &page[1].compound_pincount;
-}
-
 /*
  * Used for sizing the vmemmap region on some architectures
  */
diff --git a/mm/debug.c b/mm/debug.c
index 7f8e5f744e42..893c9dbf76ca 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -94,11 +94,11 @@ static void __dump_page(struct page *page)
 			page, page_ref_count(head), mapcount, mapping,
 			page_to_pgoff(page), page_to_pfn(page));
 	if (compound) {
-		pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d compound_pincount:%d\n",
+		pr_warn("head:%p order:%u compound_mapcount:%d subpages_mapcount:%d pincount:%d\n",
 				head, compound_order(head),
 				head_compound_mapcount(head),
 				head_subpages_mapcount(head),
-				head_compound_pincount(head));
+				atomic_read(&folio->_pincount));
 	}
 
 #ifdef CONFIG_MEMCG
diff --git a/mm/gup.c b/mm/gup.c
index f45a3a5be53a..38ba1697dd61 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -111,7 +111,7 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
  *    FOLL_GET: folio's refcount will be incremented by @refs.
  *
  *    FOLL_PIN on large folios: folio's refcount will be incremented by
- *    @refs, and its compound_pincount will be incremented by @refs.
+ *    @refs, and its pincount will be incremented by @refs.
  *
  *    FOLL_PIN on single-page folios: folio's refcount will be incremented by
  *    @refs * GUP_PIN_COUNTING_BIAS.
@@ -157,7 +157,7 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
 		 * try_get_folio() is left intact.
 		 */
 		if (folio_test_large(folio))
-			atomic_add(refs, folio_pincount_ptr(folio));
+			atomic_add(refs, &folio->_pincount);
 		else
 			folio_ref_add(folio,
 					refs * (GUP_PIN_COUNTING_BIAS - 1));
@@ -182,7 +182,7 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 	if (flags & FOLL_PIN) {
 		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
 		if (folio_test_large(folio))
-			atomic_sub(refs, folio_pincount_ptr(folio));
+			atomic_sub(refs, &folio->_pincount);
 		else
 			refs *= GUP_PIN_COUNTING_BIAS;
 	}
@@ -232,7 +232,7 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
 		 */
 		if (folio_test_large(folio)) {
 			folio_ref_add(folio, 1);
-			atomic_add(1, folio_pincount_ptr(folio));
+			atomic_add(1, &folio->_pincount);
 		} else {
 			folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
 		}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c13b1f67d14e..9570f03cdee4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2477,9 +2477,9 @@ static void __split_huge_page_tail(struct page *head, int tail,
 	 * of swap cache pages that store the swp_entry_t in tail pages.
 	 * Fix up and warn once if private is unexpectedly set.
 	 *
-	 * What of 32-bit systems, on which head[1].compound_pincount overlays
+	 * What of 32-bit systems, on which folio->_pincount overlays
 	 * head[1].private?  No problem: THP_SWAP is not enabled on 32-bit, and
-	 * compound_pincount must be 0 for folio_ref_freeze() to have succeeded.
+	 * pincount must be 0 for folio_ref_freeze() to have succeeded.
 	 */
 	if (!folio_test_swapcache(page_folio(head))) {
 		VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 273a6522aa4c..15b2707c1600 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1476,7 +1476,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
 
 	atomic_set(folio_mapcount_ptr(folio), 0);
 	atomic_set(folio_subpages_mapcount_ptr(folio), 0);
-	atomic_set(folio_pincount_ptr(folio), 0);
+	atomic_set(&folio->_pincount, 0);
 
 	for (i = 1; i < nr_pages; i++) {
 		p = folio_page(folio, i);
@@ -1998,7 +1998,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
 	}
 	atomic_set(folio_mapcount_ptr(folio), -1);
 	atomic_set(folio_subpages_mapcount_ptr(folio), 0);
-	atomic_set(folio_pincount_ptr(folio), 0);
+	atomic_set(&folio->_pincount, 0);
 	return true;
 
 out_error:
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4d9afa1048ea..d1e5ec875fd0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -775,11 +775,13 @@ void free_compound_page(struct page *page)
 
 static void prep_compound_head(struct page *page, unsigned int order)
 {
+	struct folio *folio = (struct folio *)page;
+
 	set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
 	set_compound_order(page, order);
 	atomic_set(compound_mapcount_ptr(page), -1);
 	atomic_set(subpages_mapcount_ptr(page), 0);
-	atomic_set(compound_pincount_ptr(page), 0);
+	atomic_set(&folio->_pincount, 0);
 }
 
 static void prep_compound_tail(struct page *head, int tail_idx)
@@ -1291,6 +1293,7 @@ static inline bool free_page_is_bad(struct page *page)
 
 static int free_tail_pages_check(struct page *head_page, struct page *page)
 {
+	struct folio *folio = (struct folio *)head_page;
 	int ret = 1;
 
 	/*
@@ -1314,8 +1317,8 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
 			bad_page(page, "nonzero subpages_mapcount");
 			goto out;
 		}
-		if (unlikely(head_compound_pincount(head_page))) {
-			bad_page(page, "nonzero compound_pincount");
+		if (unlikely(atomic_read(&folio->_pincount))) {
+			bad_page(page, "nonzero pincount");
 			goto out;
 		}
 		break;
-- 
2.35.1