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

[David Hildenbrand (Arm)]

On 8/21/26 16:49, 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().  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@xxxxxxxxxx>
> Closes: https://sashiko.dev/#/patchset/20260817220810.1175596-1-gourry%40gourry.net
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---
>  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


I think this is best written as:

folio = vm_normal_folio_pmd(walk->vma, addr, pmdval);
if (!folio) {
	if (is_huge_zero_pmd(pmdval)) {
		walk->action = ACTION_CONTINUE;
		return;
	}
	return;
}
That way we minimize checks on the expected path.

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David