[PATCH v2 2/2] mm/madvise: use vm_normal_folio_pmd() in cold/pageout PMD range

[Gregory Price]

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.

Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net
Cc: stable@xxxxxxxxxxxxxxx # v6.19+
Assisted-by: LLM
Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
---
 mm/madvise.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index f75a9d139980..fbb72ab49aa6 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -395,16 +395,15 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 			return 0;
 
 		orig_pmd = *pmd;
-		if (is_huge_zero_pmd(orig_pmd))
-			goto huge_unlock;
-
 		if (unlikely(!pmd_present(orig_pmd))) {
 			VM_WARN_ON_ONCE(!pmd_is_migration_entry(orig_pmd) &&
 					!pmd_is_device_private_entry(orig_pmd));
 			goto huge_unlock;
 		}
 
-		folio = pmd_folio(orig_pmd);
+		folio = vm_normal_folio_pmd(vma, addr, orig_pmd);
+		if (!folio)
+			goto huge_unlock;
 
 		if (folio_is_zone_device(folio))
 			goto huge_unlock;
-- 
2.55.0