linux-kernel - [PATCH v2 0/4] Improve folio split related functions

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Message-ID: <20251122025529.1562592-1-ziy@nvidia.com>
Date: Fri, 21 Nov 2025 21:55:25 -0500
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...nel.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>,
	Barry Song <baohua@...nel.org>,
	Lance Yang <lance.yang@...ux.dev>,
	Miaohe Lin <linmiaohe@...wei.com>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Wei Yang <richard.weiyang@...il.com>,
	Balbir Singh <balbirs@...dia.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 0/4] Improve folio split related functions

Hi all,

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().

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.

Changelog
===
>From RFC[2]
1. Renamed folio_split_supported() to folio_check_splittable(), changed
   its return type from bool to int to return error code directly, and
   added kernel-doc.

2. Moved truncated folio check, zero huge folio check, and writeback
   check in folio_check_splittable().

3. Changed zero huge folio check's error number from -EBUSY to -EINVAL.

4. Replaced can_split_folio() with open code.

5. Changed min_order_for_split() to return 0 for truncated folio instead
   of -EBUSY and added kernel-doc.

6. Fixed folio split stats counting.

Comments and feedbacks are welcome.

Link: https://lore.kernel.org/all/20251120004735.52z7r4xmogw7mbsj@master/ [1]
Link: https://lore.kernel.org/all/20251120035953.1115736-1-ziy@nvidia.com/ [2]

Zi Yan (4):
  mm/huge_memory: change folio_split_supported() to
    folio_check_splittable()
  mm/huge_memory: replace can_split_folio() with direct refcount
    calculation
  mm/huge_memory: make min_order_for_split() always return an order
  mm/huge_memory: fix folio split stats counting

 include/linux/huge_mm.h |  17 +++--
 mm/huge_memory.c        | 152 +++++++++++++++++++++++-----------------
 mm/vmscan.c             |   3 +-
 3 files changed, 101 insertions(+), 71 deletions(-)

-- 
2.51.0