Re: [PATCH v3 01/26] set_memory: add folio_{zap,restore}_direct_map helpers

[Yosry Ahmed <yosry@kernel.org>]

On Sun, Jul 26, 2026 at 10:22:34PM +0000, Brendan Jackman wrote:
> From: Nikita Kalyazin <nikita.kalyazin@linux.dev>
> 
> Let's provide folio_{zap,restore}_direct_map helpers as preparation for
> supporting removal of the direct map for guest_memfd folios.
> In folio_zap_direct_map(), flush TLB to make sure the data is not
> accessible.  On some architectures, there may be a double TLB flush
> issued because set_direct_map_valid_noflush already performs a flush
> internally.
> 
> The new helpers need to be accessible to KVM on architectures that
> support guest_memfd (x86 and arm64).
> 
> Direct map removal gives guest_memfd the same protection that
> memfd_secret does, such as hardening against Spectre-like attacks
> through in-kernel gadgets.
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Nikita Kalyazin <nikita.kalyazin@linux.dev>
> [Added comment, dropped modified set_direct_map API, added highmem check]
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
>  include/linux/set_memory.h | 13 +++++++++++++
>  mm/memory.c                | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 59 insertions(+)
> 
> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
> index 3030d9245f5ac..1bf2a15bca118 100644
> --- a/include/linux/set_memory.h
> +++ b/include/linux/set_memory.h
> @@ -40,6 +40,15 @@ static inline int set_direct_map_valid_noflush(struct page *page,
>  	return 0;
>  }
>  
> +static inline int folio_zap_direct_map(struct folio *folio)
> +{
> +	return 0;

Should this return an error (e.g. -EOPNOTSUPP)? Seems like it would
silently succeed if the arch doesn't actually support removing from the
direct map.