[PATCH v4 0/4] Improve folio split related functions

[Zi Yan <ziy@nvidia.com>]

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() 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.

The code is based on latest mm-new with V2 reverted and can replace V2
cleanly on mm-new branch.


Changelog
===
From V3[4]:
1. Reordered conditions when checking truncated shmem or to-be-anon folios.

2. Removed expected_refs.

From V2[3]:
1. Removed "bool warns" parameter from folio_check_splittable().

2. Removed all warnings in folio_check_splittable() and added a single
   warning in its caller, __folio_split() instead.

3. Spelled out in the comment in folio_check_splittable() that folios
   without a mapping in the swapcache can be shmem or to-be-anon folios.

4. Renamed folio_cache_references to folio_cache_ref_count.

5. Removed extra_pins variable.

6. Replaced folio_expected_ref_count() with folio_cache_ref_count() for
   folio_ref_unfreeze() uses in __folio_freeze_and_split_unmapped(),
   since they are equivalent at those call sites.


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]
Link: https://lore.kernel.org/all/20251122025529.1562592-1-ziy@nvidia.com/ [3]
Link: https://lore.kernel.org/all/20251126035008.1919461-1-ziy@nvidia.com/ [4]

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 |  13 ++--
 mm/huge_memory.c        | 165 ++++++++++++++++++++++------------------
 mm/vmscan.c             |   3 +-
 3 files changed, 98 insertions(+), 83 deletions(-)

-- 
2.51.0