[PATCH 1/2] mm/mempolicy: use vm_normal_folio_pmd() in queue_folios_pmd()

[Gregory Price <gourry@gourry.net>]

mmap a VM_PFNMAP region whose ->huge_fault installs a PMD through
vmf_insert_pfn_pmd() - a vfio-pci MMIO BAR does this - then

	mbind(p, len, MPOL_BIND, &mask, maxnode, MPOL_MF_STRICT);

With a stand-in module for the driver:

  BUG: unable to handle page fault for address: fffff96dc0000008
  RIP: 0010:queue_folios_pte_range+0xaf/0x440
   walk_pgd_range+0x52b/0xaf0
   __walk_page_range+0x6a/0x1d0
   walk_page_range_mm_unsafe+0x193/0x230
   queue_pages_range+0x64/0xa0
   do_mbind+0x25e/0x640

queue_folios_pmd(), inlined above, calls pmd_folio() on that PMD.  The pfn
is raw MMIO with no memmap entry, so the folio lands in unpopulated
vmemmap.  Neither guard stops the walk:

  walk_page_test()         skips VM_PFNMAP, but queue_pages_walk_ops
                           supplies ->test_walk, so it never runs
  queue_pages_test_walk()  honours vma_migratable(), but only while
                           MPOL_MF_STRICT is clear

A VM_MIXEDMAP vma needs neither flag, being vma_migratable(), so plain
mbind(MPOL_MF_MOVE) reaches this too - and there the bad folio carries on
into migrate_folio_add() and folio_isolate_lru().  mshv_vtl_low is such a
mapping.

Use vm_normal_folio_pmd() and skip on NULL, as the PTE loop in
queue_folios_pte_range() already does with vm_normal_folio().  The huge
zero PMD moves ahead of the lookup, since vm_normal_folio_pmd() returns
NULL for it and its ACTION_CONTINUE would be lost.

mbind(MPOL_MF_STRICT) over a PMD mapped VM_PFNMAP region now returns 0
rather than -EIO.  The PTE loop already returned 0 there.

Fixes: 3c8e44c9b369 ("mm: mark special bits for huge pfn mappings when inject")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 mm/mempolicy.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index f1aba551f9f1..85803c845965 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -650,7 +650,8 @@ static inline bool queue_folio_required(struct folio *folio,
 	return node_isset(nid, *qp->nmask) == !(flags & MPOL_MF_INVERT);
 }
 
-static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
+static void queue_folios_pmd(pmd_t *pmd, unsigned long addr,
+			     struct mm_walk *walk)
 {
 	struct folio *folio;
 	struct queue_pages *qp = walk->private;
@@ -661,13 +662,15 @@ static void queue_folios_pmd(pmd_t *pmd, struct mm_walk *walk)
 			qp->nr_failed++;
 		return;
 	}
-	folio = pmd_folio(pmdval);
-	if (folio_is_zone_device(folio))
-		return;
-	if (is_huge_zero_folio(folio)) {
+	if (is_huge_zero_pmd(pmdval)) {
 		walk->action = ACTION_CONTINUE;
 		return;
 	}
+	folio = vm_normal_folio_pmd(walk->vma, addr, pmdval);
+	if (!folio)
+		return;
+	if (folio_is_zone_device(folio))
+		return;
 	if (!queue_folio_required(folio, qp))
 		return;
 	if (!(qp->flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) ||
@@ -700,7 +703,7 @@ static int queue_folios_pte_range(pmd_t *pmd, unsigned long addr,
 
 	ptl = pmd_trans_huge_lock(pmd, vma);
 	if (ptl) {
-		queue_folios_pmd(pmd, walk);
+		queue_folios_pmd(pmd, addr, walk);
 		spin_unlock(ptl);
 		goto out;
 	}
-- 
2.55.0