mm/madvise: use vm_normal_folio_pmd() in cold/pageout PMD range

mmap a VM_MIXEDMAP region whose ->huge_fault installs a PMD through
vmf_insert_pfn_pmd() - mshv_vtl_low does this, and needs CAP_SYS_ADMIN
to open - then:

  madvise(p, PMD_SIZE, MADV_PAGEOUT);

With a stand-in module for the driver:

  BUG: unable to handle page fault for address: fffff587c0000008
  RIP: 0010:madvise_cold_or_pageout_pte_range+0x410/0x9b0
   walk_pgd_range+0x52b/0xaf0
   __walk_page_range+0x6a/0x1d0
   walk_page_range_vma_unsafe+0x8e/0x120
   madvise_pageout+0xb2/0x180
   madvise_vma_behavior+0x46b/0xa90
   do_madvise+0x108/0x190
   __x64_sys_madvise+0x26/0x30

Nothing validates the pfn on the way in:

  can_madv_lru_vma()     rejects VM_PFNMAP, but not VM_MIXEDMAP
  can_fault()            *pfn = vmf->pgoff & ~(mask >> PAGE_SHIFT);
  vmf_insert_pfn_pmd()   no pfn_valid() check
  pmd_folio()            pfn_to_page() -> unpopulated vmemmap

Even with a valid pfn the path is wrong.  The mapping carries no rmap, so
folio_maybe_mapped_shared() sees mapcount 0, and the walker goes on to
folio_deactivate(), or folio_isolate_lru() plus reclaim_pages(), against a
folio this mapping does not own.

Use vm_normal_folio_pmd() and skip on NULL, as the PTE half of this same
walker already does with vm_normal_folio().  This also filters the huge
zero PMD, so its separate check is no longer needed.

Link: https://lore.kernel.org/20260926105110.2156652-3-gourry@gourry.net
Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Assisted-by: LLM
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: <stable@vger.kernel.org> # v6.19+

0 files changed, 0 insertions, 0 deletions