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

[David Hildenbrand (Arm)]

On 8/21/26 16:49, Gregory Price wrote:
> 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().  The huge zero PMD is already
> handled further up by is_huge_zero_pmd().

The last part doesn't make sense, see below.

> 
> 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: Claude:claude-opus-5
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---
>  mm/madvise.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index ffd6a68320a8..ab362bc482a5 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -393,7 +393,9 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  			goto huge_unlock;
>  		}
>  
> -		folio = pmd_folio(orig_pmd);
> +		folio = vm_normal_folio_pmd(vma, addr, orig_pmd);
> +		if (!folio)
> +			goto huge_unlock;
I think you can now drop the is_huge_zero_pmd() check, because
!vm_normal_folio_pmd() includes the huge zero folio.

-- 
Cheers,

David