[RFC v2 PATCH 04/17] mm: Implement folio_add_new_anon_rmap_range()

[Ryan Roberts <ryan.roberts@arm.com>]

Like folio_add_new_anon_rmap() but batch-rmaps a range of pages
belonging to a folio, for effciency savings. All pages are accounted as
small pages.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 include/linux/rmap.h |  2 ++
 mm/rmap.c            | 43 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index b87d01660412..5c707f53d7b5 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -196,6 +196,8 @@ void page_add_new_anon_rmap(struct page *, struct vm_area_struct *,
 		unsigned long address);
 void folio_add_new_anon_rmap(struct folio *, struct vm_area_struct *,
 		unsigned long address);
+void folio_add_new_anon_rmap_range(struct folio *folio, struct page *page,
+		int nr, struct vm_area_struct *vma, unsigned long address);
 void page_add_file_rmap(struct page *, struct vm_area_struct *,
 		bool compound);
 void page_remove_rmap(struct page *, struct vm_area_struct *,
diff --git a/mm/rmap.c b/mm/rmap.c
index 8632e02661ac..d563d979c005 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1302,6 +1302,49 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
 	__page_set_anon_rmap(folio, &folio->page, vma, address, 1);
 }

+/**
+ * folio_add_new_anon_rmap_range - Add mapping to a set of pages within a new
+ * anonymous potentially large folio.
+ * @folio:      The folio containing the pages to be mapped
+ * @page:       First page in the folio to be mapped
+ * @nr:         Number of pages to be mapped
+ * @vma:        the vm area in which the mapping is added
+ * @address:    the user virtual address of the first page to be mapped
+ *
+ * Like folio_add_new_anon_rmap() but batch-maps a range of pages within a folio
+ * using non-THP accounting. Like folio_add_new_anon_rmap(), the inc-and-test is
+ * bypassed and the folio does not have to be locked. All pages in the folio are
+ * individually accounted.
+ *
+ * As the folio is new, it's assumed to be mapped exclusively by a single
+ * process.
+ */
+void folio_add_new_anon_rmap_range(struct folio *folio, struct page *page,
+		int nr, struct vm_area_struct *vma, unsigned long address)
+{
+	int i;
+
+	VM_BUG_ON_VMA(address < vma->vm_start ||
+		      address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
+	__folio_set_swapbacked(folio);
+
+	if (folio_test_large(folio)) {
+		/* increment count (starts at 0) */
+		atomic_set(&folio->_nr_pages_mapped, nr);
+	}
+
+	for (i = 0; i < nr; i++) {
+		/* increment count (starts at -1) */
+		atomic_set(&page->_mapcount, 0);
+		__page_set_anon_rmap(folio, page, vma, address, 1);
+		page++;
+		address += PAGE_SIZE;
+	}
+
+	__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
+
+}
+
 /**
  * page_add_file_rmap - add pte mapping to a file page
  * @page:	the page to add the mapping to
--
2.25.1