[RFC Patch 09/11] mm/huge_memory: introduce folio_split_uniform() helper for uniform splitting

[Wei Yang <richard.weiyang@gmail.com>]

There are two fundamental types of folio split:

    * uniform
    * non-uniform

The existing implementation uses the page-centric function
__split_huge_page_to_list_to_order() to handle uniform splits.

This commit introduces a new, dedicated folio-style API,
folio_split_uniform(), which wraps the existing split logic. This
transition improves consistency by adopting the modern folio API and
paves the way for the eventual removal of the older, page-centric
interface.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
---
 include/linux/huge_mm.h |  1 +
 mm/huge_memory.c        | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index cf38ed6b9835..bc99ae7b0376 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -376,6 +376,7 @@ unsigned int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
 int folio_check_splittable(struct folio *folio, unsigned int new_order,
 			   enum split_type split_type);
+int folio_split_uniform(struct folio *folio, unsigned int new_order, struct list_head *list);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index a8ca7c5902f4..09742c021eee 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4199,6 +4199,24 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
 			     SPLIT_TYPE_UNIFORM);
 }
 
+/**
+ * folio_split_uniform() - split a folio at @split_at to a @new_order folio
+ * @folio: folio to split
+ * @new_order: the order of the new folio
+ * @list: after-split folios are added to @list if not null, otherwise to LRU
+ *        list
+ *
+ * After split, folio is left locked for caller.
+ *
+ * Return: 0 - successful, <0 - failed (if -ENOMEM is returned, @folio might be
+ * split but not to @new_order, the caller needs to check)
+ */
+int folio_split_uniform(struct folio *folio, unsigned int new_order, struct list_head *list)
+{
+	return __folio_split(folio, new_order, NULL, &folio->page, list,
+			     SPLIT_TYPE_UNIFORM);
+}
+
 /**
  * folio_split() - split a folio at @split_at to a @new_order folio
  * @folio: folio to split
-- 
2.34.1