mm/huge_memory: change folio_split_supported() to folio_check_splittable()

Patch series "Improve folio split related functions", v4.

This patchset improves several folio split related functions to avoid
future misuse.  The changes are:

1. Consolidated folio splittable checks by moving truncated folio check,
   huge zero folio check, and writeback folio check into
   folio_split_supported(). Changed the function return type. Renamed it
   to folio_check_splittable() for clarification.

2. Replaced can_split_folio() with open coded folio_expected_ref_count()
   and folio_ref_count() and introduced folio_cache_ref_count().

3. Changed min_order_for_split() to always return an order.

4. Fixed folio split stats counting.

Motivation
==========
This is based on Wei's observation[1] and solves several potential
issues:
1. Dereferencing NULL folio->mapping in try_folio_split_to_order() if it
   is called on truncated folios.
2. Not handling of negative return value of min_order_for_split() in
   mm/memory-failure.c

There is no bug in the current code.


This patch (of 4):

folio_split_supported() used in try_folio_split_to_order() requires
folio->mapping to be non NULL, but current try_folio_split_to_order() does
not check it.  There is no issue in the current code, since
try_folio_split_to_order() is only used in truncate_inode_partial_folio(),
where folio->mapping is not NULL.

To prevent future misuse, move folio->mapping NULL check (i.e., folio is
truncated) into folio_split_supported().  Since folio->mapping NULL check
returns -EBUSY and folio_split_supported() == false means -EINVAL, change
folio_split_supported() return type from bool to int and return error
numbers accordingly.  Rename folio_split_supported() to
folio_check_splittable() to match the return type change.

While at it, move is_huge_zero_folio() check and folio_test_writeback()
check into folio_check_splittable() and add kernel-doc.

Remove all warnings inside folio_check_splittable() and give warnings
in __folio_split() instead, so that bool warns parameter can be removed.

Link: https://lkml.kernel.org/r/20251126210618.1971206-1-ziy@nvidia.com
Link: https://lkml.kernel.org/r/20251126210618.1971206-2-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 1d439de1ca2c..66105a90b4c3 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -375,8 +375,8 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list
 int folio_split_unmapped(struct folio *folio, unsigned int new_order);
 int min_order_for_split(struct folio *folio);
 int split_folio_to_list(struct folio *folio, struct list_head *list);
-bool folio_split_supported(struct folio *folio, unsigned int new_order,
-		enum split_type split_type, bool warns);
+int folio_check_splittable(struct folio *folio, unsigned int new_order,
+			   enum split_type split_type);
 int folio_split(struct folio *folio, unsigned int new_order, struct page *page,
 		struct list_head *list);
 
@@ -407,7 +407,7 @@ static inline int split_huge_page_to_order(struct page *page, unsigned int new_o
 static inline int try_folio_split_to_order(struct folio *folio,
 		struct page *page, unsigned int new_order)
 {
-	if (!folio_split_supported(folio, new_order, SPLIT_TYPE_NON_UNIFORM, /* warns= */ false))
+	if (folio_check_splittable(folio, new_order, SPLIT_TYPE_NON_UNIFORM))
 		return split_huge_page_to_order(&folio->page, new_order);
 	return folio_split(folio, new_order, page, NULL);
 }