[PATCH v3 1/7] mm: Add folio_split_unref helper

[Francois Dugast <francois.dugast@intel.com>]

From: Matthew Brost <matthew.brost@intel.com>

Add folio_split_unref helper which splits an unreferenced folio
(refcount == 0) into individual pages. Intended to be called on special
pages (e.g., device-private, DAX, etc.) when returning the folio to the
free page pool.

Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Suggested-by: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
---
 include/linux/huge_mm.h |  1 +
 mm/huge_memory.c        | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index a4d9f964dfde..18cb9728d8f1 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -369,6 +369,7 @@ enum split_type {
 	SPLIT_TYPE_NON_UNIFORM,
 };
 
+void folio_split_unref(struct folio *folio);
 int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
 		unsigned int new_order);
 int folio_split_unmapped(struct folio *folio, unsigned int new_order);
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 40cf59301c21..0eb9e6ad8639 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3580,6 +3580,45 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
 		ClearPageCompound(&folio->page);
 }
 
+/**
+ * folio_split_unref() - split an unreferenced folio (refcount == 0)
+ * @folio: the to-be-split folio
+ *
+ * Split an unreferenced folio (refcount == 0) into individual pages.
+ * Intended to be called on special pages (e.g., device-private, DAX, etc.)
+ * when returning the folio to the free page pool.
+ */
+void folio_split_unref(struct folio *folio)
+{
+	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
+	int order, i;
+
+	folio->mapping = NULL;
+	order = folio_order(folio);
+	if (!order)
+		return;
+
+	folio_reset_order(folio);
+
+	for (i = 0; i < (1UL << order); i++) {
+		struct page *page = folio_page(folio, i);
+		struct folio *new_folio = (struct folio *)page;
+
+		ClearPageHead(page);
+		clear_compound_head(page);
+
+		new_folio->mapping = NULL;
+		/*
+		 * Reset pgmap which was over-written by
+		 * prep_compound_page().
+		 */
+		new_folio->pgmap = pgmap;
+		new_folio->share = 0;	/* fsdax only, unused for device private */
+		VM_WARN_ON_FOLIO(folio_ref_count(new_folio), new_folio);
+	}
+}
+EXPORT_SYMBOL_GPL(folio_split_unref);
+
 /**
  * __split_unmapped_folio() - splits an unmapped @folio to lower order folios in
  * two ways: uniform split or non-uniform split.
-- 
2.43.0