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

[Mike Rapoport <rppt@kernel.org>]

On Thu, Jul 30, 2026 at 08:34:57PM +0000, Yosry Ahmed wrote:
> 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.

That's the pattern we have now for all set_memory APIs.

-- 
Sincerely yours,
Mike.