[PATCH 1/2] mm: Turn folio_test_hugetlb into a PageType

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

The current folio_test_hugetlb() can be fooled by a concurrent folio split
into returning true for a folio which has never belonged to hugetlbfs.
This can't happen if the caller holds a refcount on it, but we have a
few places (memory-failure, compaction, procfs) which do not and should
not take a speculative reference.

Since hugetlb pages do not use individual page mapcounts (they are always
fully mapped and use the entire_mapcount field to record the number of
mappings), the PageType field is available for use with only a minor
modification to page_mapcount() to ignore the value in this field for
hugetlb pages.  Other parts of the kernel which examine page->_mapcount
directly already know about page_has_type() (eg procfs, dump_page()).

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/mm.h         |  3 ++
 include/linux/page-flags.h | 70 ++++++++++++++++++--------------------
 mm/hugetlb.c               | 22 ++----------
 3 files changed, 39 insertions(+), 56 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f5a97dec5169..32281097f07e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1212,6 +1212,9 @@ static inline int page_mapcount(struct page *page)
 {
 	int mapcount = atomic_read(&page->_mapcount) + 1;
 
+	/* Head page repurposes mapcount; tail page leaves at -1 */
+	if (PageHeadHuge(page))
+		mapcount = 0;
 	if (unlikely(PageCompound(page)))
 		mapcount += folio_entire_mapcount(page_folio(page));
 
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 735cddc13d20..6ebb573c7195 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -190,7 +190,6 @@ enum pageflags {
 
 	/* At least one page in this folio has the hwpoison flag set */
 	PG_has_hwpoisoned = PG_error,
-	PG_hugetlb = PG_active,
 	PG_large_rmappable = PG_workingset, /* anon or file-backed */
 };
 
@@ -829,29 +828,6 @@ TESTPAGEFLAG_FALSE(LargeRmappable, large_rmappable)
 
 #define PG_head_mask ((1UL << PG_head))
 
-#ifdef CONFIG_HUGETLB_PAGE
-int PageHuge(struct page *page);
-SETPAGEFLAG(HugeTLB, hugetlb, PF_SECOND)
-CLEARPAGEFLAG(HugeTLB, hugetlb, PF_SECOND)
-
-/**
- * folio_test_hugetlb - Determine if the folio belongs to hugetlbfs
- * @folio: The folio to test.
- *
- * Context: Any context.  Caller should have a reference on the folio to
- * prevent it from being turned into a tail page.
- * Return: True for hugetlbfs folios, false for anon folios or folios
- * belonging to other filesystems.
- */
-static inline bool folio_test_hugetlb(struct folio *folio)
-{
-	return folio_test_large(folio) &&
-		test_bit(PG_hugetlb, folio_flags(folio, 1));
-}
-#else
-TESTPAGEFLAG_FALSE(Huge, hugetlb)
-#endif
-
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 /*
  * PageHuge() only returns true for hugetlbfs pages, but not for
@@ -907,18 +883,6 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
 	TESTSCFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
 #endif
 
-/*
- * Check if a page is currently marked HWPoisoned. Note that this check is
- * best effort only and inherently racy: there is no way to synchronize with
- * failing hardware.
- */
-static inline bool is_page_hwpoison(struct page *page)
-{
-	if (PageHWPoison(page))
-		return true;
-	return PageHuge(page) && PageHWPoison(compound_head(page));
-}
-
 /*
  * For pages that are never mapped to userspace (and aren't PageSlab),
  * page_type may be used.  Because it is initialised to -1, we invert the
@@ -935,6 +899,7 @@ static inline bool is_page_hwpoison(struct page *page)
 #define PG_offline	0x00000100
 #define PG_table	0x00000200
 #define PG_guard	0x00000400
+#define PG_hugetlb	0x00000800
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
@@ -1026,6 +991,37 @@ PAGE_TYPE_OPS(Table, table, pgtable)
  */
 PAGE_TYPE_OPS(Guard, guard, guard)
 
+#ifdef CONFIG_HUGETLB_PAGE
+PAGE_TYPE_OPS(HeadHuge, hugetlb, hugetlb)
+#else
+TESTPAGEFLAG_FALSE(HeadHuge, hugetlb)
+#endif
+
+/**
+ * PageHuge - Determine if the page belongs to hugetlbfs
+ * @page: The page to test.
+ *
+ * Context: Any context.
+ * Return: True for hugetlbfs pages, false for anon pages or pages
+ * belonging to other filesystems.
+ */
+static inline bool PageHuge(struct page *page)
+{
+	return folio_test_hugetlb(page_folio(page));
+}
+
+/*
+ * Check if a page is currently marked HWPoisoned. Note that this check is
+ * best effort only and inherently racy: there is no way to synchronize with
+ * failing hardware.
+ */
+static inline bool is_page_hwpoison(struct page *page)
+{
+	if (PageHWPoison(page))
+		return true;
+	return PageHuge(page) && PageHWPoison(compound_head(page));
+}
+
 extern bool is_free_buddy_page(struct page *page);
 
 PAGEFLAG(Isolated, isolated, PF_ANY);
@@ -1092,7 +1088,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 #define PAGE_FLAGS_SECOND						\
 	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
-	 1UL << PG_hugetlb		| 1UL << PG_large_rmappable)
+	 1UL << PG_large_rmappable)
 
 #define PAGE_FLAGS_PRIVATE				\
 	(1UL << PG_private | 1UL << PG_private_2)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ed1581b670d4..23b62df21971 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1623,7 +1623,7 @@ static inline void __clear_hugetlb_destructor(struct hstate *h,
 {
 	lockdep_assert_held(&hugetlb_lock);
 
-	folio_clear_hugetlb(folio);
+	__folio_clear_hugetlb(folio);
 }
 
 /*
@@ -1710,7 +1710,7 @@ static void add_hugetlb_folio(struct hstate *h, struct folio *folio,
 		h->surplus_huge_pages_node[nid]++;
 	}
 
-	folio_set_hugetlb(folio);
+	__folio_set_hugetlb(folio);
 	folio_change_private(folio, NULL);
 	/*
 	 * We have to set hugetlb_vmemmap_optimized again as above
@@ -2048,7 +2048,7 @@ static void __prep_account_new_huge_page(struct hstate *h, int nid)
 
 static void init_new_hugetlb_folio(struct hstate *h, struct folio *folio)
 {
-	folio_set_hugetlb(folio);
+	__folio_set_hugetlb(folio);
 	INIT_LIST_HEAD(&folio->lru);
 	hugetlb_set_folio_subpool(folio, NULL);
 	set_hugetlb_cgroup(folio, NULL);
@@ -2158,22 +2158,6 @@ static bool prep_compound_gigantic_folio_for_demote(struct folio *folio,
 	return __prep_compound_gigantic_folio(folio, order, true);
 }
 
-/*
- * PageHuge() only returns true for hugetlbfs pages, but not for normal or
- * transparent huge pages.  See the PageTransHuge() documentation for more
- * details.
- */
-int PageHuge(struct page *page)
-{
-	struct folio *folio;
-
-	if (!PageCompound(page))
-		return 0;
-	folio = page_folio(page);
-	return folio_test_hugetlb(folio);
-}
-EXPORT_SYMBOL_GPL(PageHuge);
-
 /*
  * Find and lock address space (mapping) in write mode.
  *
-- 
2.43.0