hugetlb: remove PageHeadHuge()

Sidhartha Kumar removed the last caller of PageHeadHuge(), so we can now
remove it and make folio_test_hugetlb() the real implementation.  Add
kernel-doc for folio_test_hugetlb().

Link: https://lkml.kernel.org/r/20230327151050.1787744-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

include/linux/page-flags.h

@@ -815,14 +815,9 @@ static inline void ClearPageCompound(struct page *page)
 
 #ifdef CONFIG_HUGETLB_PAGE
 int PageHuge(struct page *page);
-int PageHeadHuge(struct page *page);
-static inline bool folio_test_hugetlb(struct folio *folio)
-{
-	return PageHeadHuge(&folio->page);
-}
+bool folio_test_hugetlb(struct folio *folio);
 #else
 TESTPAGEFLAG_FALSE(Huge, hugetlb)
-TESTPAGEFLAG_FALSE(HeadHuge, headhuge)
 #endif
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE

mm/hugetlb.c

@@ -2050,19 +2050,23 @@ int PageHuge(struct page *page)
 }
 EXPORT_SYMBOL_GPL(PageHuge);
 
-/*
- * PageHeadHuge() only returns true for hugetlbfs head page, but not for
- * normal or transparent huge pages.
+/**
+ * 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.
  */
-int PageHeadHuge(struct page *page_head)
+bool folio_test_hugetlb(struct folio *folio)
 {
-	struct folio *folio = (struct folio *)page_head;
 	if (!folio_test_large(folio))
-		return 0;
+		return false;
 
 	return folio->_folio_dtor == HUGETLB_PAGE_DTOR;
 }
-EXPORT_SYMBOL_GPL(PageHeadHuge);
+EXPORT_SYMBOL_GPL(folio_test_hugetlb);
 
 /*
  * Find and lock address space (mapping) in write mode.