Based on the prior discussion[1], this patch improves
__split_unmapped_folio() by making it reusable for splitting unmapped folios.
This helps avoid the need for a new boolean unmapped parameter to guard
mapping-related code.

An additional benefit is that __split_unmapped_folio() could be
called on after-split folios by __folio_split(). It can enable new split
methods. For example, at deferred split time, unmapped subpages can
scatter arbitrarily within a large folio, neither uniform nor non-uniform
split can maximize after-split folio orders for mapped subpages.
The hope is that by calling __split_unmapped_folio() multiple times,
a better split result can be achieved.

It passed mm selftests.


Changelog
===
From V1[2]:
1. Fixed indentations.
2. Used folio_expected_ref_count() to calculate ref_count instead of
   open coding.


[1] https://lore.kernel.org/linux-mm/94D8C1A4-780C-4BEC-A336-7D3613B54845@nvidia.com/
[2] https://lore.kernel.org/linux-mm/20250711030259.3574392-1-ziy@nvidia.com/

Zi Yan (2):
  mm/huge_memory: move unrelated code out of __split_unmapped_folio()
  mm/huge_memory: use folio_expected_ref_count() to calculate ref_count.

 mm/huge_memory.c | 271 +++++++++++++++++++++++------------------------
 1 file changed, 135 insertions(+), 136 deletions(-)

-- 
2.47.2