mm: Make copy_huge_page() always available

Rewrite copy_huge_page() and move it into mm/util.c so it's always
available.  Fixes an exposure of uninitialised memory on configurations
with HUGETLB and UFFD enabled and MIGRATION disabled.

Fixes: 8cc5fcbb5be8 ("mm, hugetlb: fix racy resv_huge_pages underflow on UFFDIO_COPY")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 9b7b7cd..23dadf7 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -51,7 +51,6 @@
 				  struct page *newpage, struct page *page);
 extern int migrate_page_move_mapping(struct address_space *mapping,
 		struct page *newpage, struct page *page, int extra_count);
-extern void copy_huge_page(struct page *dst, struct page *src);
 #else
 
 static inline void putback_movable_pages(struct list_head *l) {}
@@ -77,10 +76,6 @@
 {
 	return -ENOSYS;
 }
-
-static inline void copy_huge_page(struct page *dst, struct page *src)
-{
-}
 #endif /* CONFIG_MIGRATION */
 
 #ifdef CONFIG_COMPACTION
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 57453db..7ca22e6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -906,6 +906,7 @@
 void put_pages_list(struct list_head *pages);
 
 void split_page(struct page *page, unsigned int order);
+void copy_huge_page(struct page *dst, struct page *src);
 
 /*
  * Compound pages have a destructor function.  Provide a
diff --git a/mm/migrate.c b/mm/migrate.c
index 23cbd9d..34a9ad3 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -537,54 +537,6 @@
 }
 
 /*
- * Gigantic pages are so large that we do not guarantee that page++ pointer
- * arithmetic will work across the entire page.  We need something more
- * specialized.
- */
-static void __copy_gigantic_page(struct page *dst, struct page *src,
-				int nr_pages)
-{
-	int i;
-	struct page *dst_base = dst;
-	struct page *src_base = src;
-
-	for (i = 0; i < nr_pages; ) {
-		cond_resched();
-		copy_highpage(dst, src);
-
-		i++;
-		dst = mem_map_next(dst, dst_base, i);
-		src = mem_map_next(src, src_base, i);
-	}
-}
-
-void copy_huge_page(struct page *dst, struct page *src)
-{
-	int i;
-	int nr_pages;
-
-	if (PageHuge(src)) {
-		/* hugetlbfs page */
-		struct hstate *h = page_hstate(src);
-		nr_pages = pages_per_huge_page(h);
-
-		if (unlikely(nr_pages > MAX_ORDER_NR_PAGES)) {
-			__copy_gigantic_page(dst, src, nr_pages);
-			return;
-		}
-	} else {
-		/* thp page */
-		BUG_ON(!PageTransHuge(src));
-		nr_pages = thp_nr_pages(src);
-	}
-
-	for (i = 0; i < nr_pages; i++) {
-		cond_resched();
-		copy_highpage(dst + i, src + i);
-	}
-}
-
-/*
  * Copy the page to its new location
  */
 void migrate_page_states(struct page *newpage, struct page *page)
diff --git a/mm/util.c b/mm/util.c
index 99c6cc7..9043d03 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -731,6 +731,16 @@
 }
 EXPORT_SYMBOL_GPL(__page_mapcount);
 
+void copy_huge_page(struct page *dst, struct page *src)
+{
+	unsigned i, nr = compound_nr(src);
+
+	for (i = 0; i < nr; i++) {
+		cond_resched();
+		copy_highpage(nth_page(dst, i), nth_page(src, i));
+	}
+}
+
 int sysctl_overcommit_memory __read_mostly = OVERCOMMIT_GUESS;
 int sysctl_overcommit_ratio __read_mostly = 50;
 unsigned long sysctl_overcommit_kbytes __read_mostly;