mm/huge_memory: move anon_vma handling into the anon split helper

Only anon split needs the anon_vma, and it only needs it to unmap and
remap.  Move the folio_get_anon_vma()/anon_vma_lock_write() pair out of
__folio_split() into the anon helper next to the folio_mapped() check that
already gates unmap_folio().

This makes the anon_vma conditional on folio_mapped(), which is a
behaviour change but should be fine.  folio_get_anon_vma() returns NULL
whenever !folio_mapped(), so an anon folio with folio_mapcount() == 0 used
to get -EBUSY from split_huge_page() and is now split instead.  A
realistic case is a THP that has been fully swapped out and is still in
the swap cache: swap PTEs do not contribute mapcount, so it is
!folio_mapped() but still alive.

That should be safe and right to have because:

- folio_ref_freeze() below still rejects a folio that picked up any
  reference, a mapping or a GUP pin, in the meantime.

- A parallel split is excluded by the folio lock.  The anon_vma
  write lock was added to serialize split in commit 062f1af2170a
  ("mm: thp: acquire the anon_vma rwsem for write during split"), when
  split_huge_page() did not hold the folio lock throughout. commit
  e9b61f19858a ("thp: reintroduce split_huge_page()") later made the
  folio lock a caller requirement and added the folio_ref_freeze()
  scheme, so that has been covered ever since.

- Unmapped path is already exercised by folio_split_unmapped(),
  and the swap cache split already runs well for a partially
  swapped-out mapped THP.

- A !folio_mapped() folio cannot become mapped meanwhile: mapping it
  requires the folio lock.

For mapped folios the anon_vma write lock is now released before
__folio_split() unlocks the after-split sub-folios, where previously it
was held across that loop; that window is harmless as the sub-folios stay
folio-locked and referenced until it.

Link: https://lore.kernel.org/20260923-swap-thp-cleanup-v6-12-ba1b4ba72c6f@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Shivam Kalra <shivamkalra98@zohomail.in>
Cc: Kairui Song <ryncsn@gmail.com>

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