[RFC v2 PATCH 13/17] mm: Implement folio_remove_rmap_range()

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

Like page_remove_rmap() but batch-removes the rmap for 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            | 62 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 8cb0ba48d58f..7daf25887049 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -204,6 +204,8 @@ void page_add_file_rmap(struct page *, struct vm_area_struct *,
 		bool compound);
 void page_remove_rmap(struct page *, struct vm_area_struct *,
 		bool compound);
+void folio_remove_rmap_range(struct folio *folio, struct page *page,
+		int nr, struct vm_area_struct *vma);

 void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *,
 		unsigned long address, rmap_t flags);
diff --git a/mm/rmap.c b/mm/rmap.c
index 1cd8fb0b929f..954e44054d5c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1419,6 +1419,68 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
 	mlock_vma_folio(folio, vma, compound);
 }

+/**
+ * folio_remove_rmap_range - take down pte mappings from a range of pages
+ * belonging to a folio. All pages are accounted as small pages.
+ * @folio:	folio that all pages belong to
+ * @page:       first page in range to remove mapping from
+ * @nr:		number of pages in range to remove mapping from
+ * @vma:        the vm area from which the mapping is removed
+ *
+ * The caller needs to hold the pte lock.
+ */
+void folio_remove_rmap_range(struct folio *folio, struct page *page,
+					int nr, struct vm_area_struct *vma)
+{
+	atomic_t *mapped = &folio->_nr_pages_mapped;
+	int nr_unmapped = 0;
+	int nr_mapped;
+	bool last;
+	enum node_stat_item idx;
+
+	VM_BUG_ON_FOLIO(folio_test_hugetlb(folio), folio);
+
+	if (!folio_test_large(folio)) {
+		/* Is this the page's last map to be removed? */
+		last = atomic_add_negative(-1, &page->_mapcount);
+		nr_unmapped = last;
+	} else {
+		for (; nr != 0; nr--, page++) {
+			/* Is this the page's last map to be removed? */
+			last = atomic_add_negative(-1, &page->_mapcount);
+			if (last) {
+				/* Page still mapped if folio mapped entirely */
+				nr_mapped = atomic_dec_return_relaxed(mapped);
+				if (nr_mapped < COMPOUND_MAPPED)
+					nr_unmapped++;
+			}
+		}
+	}
+
+	if (nr_unmapped) {
+		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
+		__lruvec_stat_mod_folio(folio, idx, -nr_unmapped);
+
+		/*
+		 * Queue anon THP for deferred split if we have just unmapped at
+		 * least 1 page, while at least 1 page remains mapped.
+		 */
+		if (folio_test_large(folio) && folio_test_anon(folio))
+			if (nr_mapped)
+				deferred_split_folio(folio);
+	}
+
+	/*
+	 * It would be tidy to reset folio_test_anon mapping when fully
+	 * unmapped, but that might overwrite a racing page_add_anon_rmap
+	 * which increments mapcount after us but sets mapping before us:
+	 * so leave the reset to free_pages_prepare, and remember that
+	 * it's only reliable while mapped.
+	 */
+
+	munlock_vma_folio(folio, vma, false);
+}
+
 /**
  * page_remove_rmap - take down pte mapping from a page
  * @page:	page to remove mapping from
--
2.25.1