Re: [PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start()

[David Hildenbrand]

On 09.08.24 18:08, Peter Xu wrote:
> Pfnmaps can always be identified with special bits in the ptes/pmds/puds.
> However that's unnecessary if the vma is stable, and when it's mapped under
> VM_PFNMAP | VM_IO.
>
> Instead of adding similar checks in all the levels for huge pfnmaps, let
> folio_walk_start() fail even earlier for these mappings.  It's also
> something gup-slow already does, so make them match.
>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>   mm/pagewalk.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
> index cd79fb3b89e5..fd3965efe773 100644
> --- a/mm/pagewalk.c
> +++ b/mm/pagewalk.c
> @@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw,
>   	p4d_t *p4dp;
>   
>   	mmap_assert_locked(vma->vm_mm);
> +
> +	/* It has no folio backing the mappings at all.. */
> +	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
> +		return NULL;
> +

That is in general not what we want, and we still have some places that 
wrongly hard-code that behavior.

In a MAP_PRIVATE mapping you might have anon pages that we can happily walk.

vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO, 
vm_normal_page_pud()] should be able to identify PFN maps and reject 
them, no?

--
Cheers,

David / dhildenb