selftests/mm: remove hardcoded THP sizing assumptions in hmm tests

selftests/mm: remove hardcoded THP sizing assumptions in hmm tests

migrate_partial_unmap_fault() and migrate_remap_fault() use hardcoded
offsets based on a 2MB PMD size.  Similarly, benchmark_thp_migration()
assumes a fixed 2MB THP size when generating test buffer sizes.

Derive offsets and test sizes from the runtime PMD page size returned by
read_pmd_pagesize().  If unavailable, fall back to TWOMEG.  This allows
the tests to adapt correctly on systems where PMD-sized THP differs from
2MB.  Also replace the fixed 1MB unmap size with a PMD-relative value
derived from the runtime PMD size.

On systems with larger PMD sizes, computed test buffer sizes can exceed
INT_MAX.  Skip such test cases to avoid overflow.

Link: https://lore.kernel.org/20260611034102.1030738-3-aboorvad@linux.ibm.com
Fixes: 24c2c5b8ffbd ("selftests/mm/hmm-tests: partial unmap, mremap and anon_write tests")
Fixes: 271a7b2e3c13 ("selftests/mm/hmm-tests: new throughput tests including THP")
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>