Re: [PATCH v2 1/2] mm/mempolicy: use vm_normal_folio_pmd() in queue_folios_pmd()

["Zi Yan" <ziy@nvidia.com>]

On Fri Sep 11, 2026 at 11:48 PM EDT, Gregory Price wrote:
> 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().  On the NULL
> path, retain ACTION_CONTINUE handling for the huge zero PMD.
>
> 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: LLM
> Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
>  mm/mempolicy.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
LGTM.

Reviewed-by: Zi Yan <ziy@nvidia.com>

-- 
Best Regards,
Yan, Zi