selftests/mm: split_huge_page_test: fix occasional is_backed_by_folio() wrong results

Patch series "selftests/mm: split_huge_page_test: split_pte_mapped_thp
improvements", v2.

One fix for occasional failures I found while testing and a bunch of
cleanups that should make that test easier to digest.


This patch (of 2):

When checking for actual tail or head pages of a folio, we must make sure
that the KPF_COMPOUND_HEAD/KPF_COMPOUND_TAIL flag is paired with KPF_THP.

For example, if we have another large folio after our large folio in
physical memory, our "pfn_flags & (KPF_THP | KPF_COMPOUND_TAIL)" would
trigger even though it's actually a head page of the next folio.

If is_backed_by_folio() returns a wrong result, split_pte_mapped_thp() can
fail with "Some THPs are missing during mremap".

Fix it by checking for head/tail pages of folios properly.  Add
folio_tail_flags/folio_head_flags to improve readability and use these
masks also when just testing for any compound page.

Link: https://lkml.kernel.org/r/20250903070253.34556-1-david@redhat.com
Link: https://lkml.kernel.org/r/20250903070253.34556-2-david@redhat.com
Fixes: 169b456b0162 ("selftests/mm: reimplement is_backed_by_thp() with more precise check")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mariano Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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