[PATCH v2 20/29] mm: convert "movable" flag in page->mapping to a page flag

[David Hildenbrand <david@redhat.com>]

Instead, let's use a page flag. As the page flag can result in
false-positives, glue it to the page types for which we
support/implement movable_ops page migration.

We are reusing PG_uptodate, that is for example used to track file
system state and does not apply to movable_ops pages. So
warning in case it is set in page_has_movable_ops() on other page types
could result in false-positive warnings.

Likely we could set the bit using a non-atomic update: in contrast to
page->mapping, we could have others trying to update the flags
concurrently when trying to lock the folio. In
isolate_movable_ops_page(), we already take care of that by checking if
the page has movable_ops before locking it. Let's start with the atomic
variant, we could later switch to the non-atomic variant once we are
sure other cases are similarly fine. Once we perform the switch, we'll
have to introduce __SETPAGEFLAG_NOOP().

Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/balloon_compaction.h |  2 +-
 include/linux/migrate.h            |  8 -----
 include/linux/page-flags.h         | 54 ++++++++++++++++++++++++------
 mm/compaction.c                    |  6 ----
 mm/zpdesc.h                        |  2 +-
 5 files changed, 46 insertions(+), 26 deletions(-)

diff --git a/include/linux/balloon_compaction.h b/include/linux/balloon_compaction.h
index a8a1706cc56f3..b222b0737c466 100644
--- a/include/linux/balloon_compaction.h
+++ b/include/linux/balloon_compaction.h
@@ -92,7 +92,7 @@ static inline void balloon_page_insert(struct balloon_dev_info *balloon,
 				       struct page *page)
 {
 	__SetPageOffline(page);
-	__SetPageMovable(page);
+	SetPageMovableOps(page);
 	set_page_private(page, (unsigned long)balloon);
 	list_add(&page->lru, &balloon->pages);
 }
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 6aece3f3c8be8..acadd41e0b5cf 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -103,14 +103,6 @@ static inline int migrate_huge_page_move_mapping(struct address_space *mapping,
 
 #endif /* CONFIG_MIGRATION */
 
-#ifdef CONFIG_COMPACTION
-void __SetPageMovable(struct page *page);
-#else
-static inline void __SetPageMovable(struct page *page)
-{
-}
-#endif
-
 #ifdef CONFIG_NUMA_BALANCING
 int migrate_misplaced_folio_prepare(struct folio *folio,
 		struct vm_area_struct *vma, int node);
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4c27ebb689e3c..5f2b570735852 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -170,6 +170,11 @@ enum pageflags {
 	/* non-lru isolated movable page */
 	PG_isolated = PG_reclaim,
 
+#ifdef CONFIG_MIGRATION
+	/* this is a movable_ops page (for selected typed pages only) */
+	PG_movable_ops = PG_uptodate,
+#endif
+
 	/* Only valid for buddy pages. Used to track pages that are reported */
 	PG_reported = PG_uptodate,
 
@@ -698,9 +703,6 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
  * bit; and then folio->mapping points, not to an anon_vma, but to a private
  * structure which KSM associates with that merged page.  See ksm.h.
  *
- * PAGE_MAPPING_KSM without PAGE_MAPPING_ANON is used for non-lru movable
- * page and then folio->mapping points to a struct movable_operations.
- *
  * Please note that, confusingly, "folio_mapping" refers to the inode
  * address_space which maps the folio from disk; whereas "folio_mapped"
  * refers to user virtual address space into which the folio is mapped.
@@ -743,13 +745,6 @@ static __always_inline bool PageAnon(const struct page *page)
 {
 	return folio_test_anon(page_folio(page));
 }
-
-static __always_inline bool page_has_movable_ops(const struct page *page)
-{
-	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) ==
-				PAGE_MAPPING_MOVABLE;
-}
-
 #ifdef CONFIG_KSM
 /*
  * A KSM page is one of those write-protected "shared pages" or "merged pages"
@@ -1133,6 +1128,45 @@ bool is_free_buddy_page(const struct page *page);
 
 PAGEFLAG(Isolated, isolated, PF_ANY);
 
+#ifdef CONFIG_MIGRATION
+/*
+ * This page is migratable through movable_ops (for selected typed pages
+ * only).
+ *
+ * Page migration of such pages might fail, for example, if the page is
+ * already isolated by somebody else, or if the page is about to get freed.
+ *
+ * While a subsystem might set selected typed pages that support page migration
+ * as being movable through movable_ops, it must never clear this flag.
+ *
+ * This flag is only cleared when the page is freed back to the buddy.
+ *
+ * Only selected page types support this flag (see page_movable_ops()) and
+ * the flag might be used in other context for other pages. Always use
+ * page_has_movable_ops() instead.
+ */
+TESTPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
+SETPAGEFLAG(MovableOps, movable_ops, PF_NO_TAIL);
+#else /* !CONFIG_MIGRATION */
+TESTPAGEFLAG_FALSE(MovableOps, movable_ops);
+SETPAGEFLAG_NOOP(MovableOps, movable_ops);
+#endif /* CONFIG_MIGRATION */
+
+/**
+ * page_has_movable_ops - test for a movable_ops page
+ * @page The page to test.
+ *
+ * Test whether this is a movable_ops page. Such pages will stay that
+ * way until freed.
+ *
+ * Returns true if this is a movable_ops page, otherwise false.
+ */
+static inline bool page_has_movable_ops(const struct page *page)
+{
+	return PageMovableOps(page) &&
+	       (PageOffline(page) || PageZsmalloc(page));
+}
+
 static __always_inline int PageAnonExclusive(const struct page *page)
 {
 	VM_BUG_ON_PGFLAGS(!PageAnon(page), page);
diff --git a/mm/compaction.c b/mm/compaction.c
index 348eb754cb227..349f4ea0ec3e5 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -114,12 +114,6 @@ static unsigned long release_free_list(struct list_head *freepages)
 }
 
 #ifdef CONFIG_COMPACTION
-void __SetPageMovable(struct page *page)
-{
-	VM_BUG_ON_PAGE(!PageLocked(page), page);
-	page->mapping = (void *)(PAGE_MAPPING_MOVABLE);
-}
-EXPORT_SYMBOL(__SetPageMovable);
 
 /* Do not skip compaction more than 64 times */
 #define COMPACT_MAX_DEFER_SHIFT 6
diff --git a/mm/zpdesc.h b/mm/zpdesc.h
index 6855d9e2732d8..25bf5ea0beb83 100644
--- a/mm/zpdesc.h
+++ b/mm/zpdesc.h
@@ -154,7 +154,7 @@ static inline struct zpdesc *pfn_zpdesc(unsigned long pfn)
 
 static inline void __zpdesc_set_movable(struct zpdesc *zpdesc)
 {
-	__SetPageMovable(zpdesc_page(zpdesc));
+	SetPageMovableOps(zpdesc_page(zpdesc));
 }
 
 static inline void __zpdesc_set_zsmalloc(struct zpdesc *zpdesc)
-- 
2.49.0