Re: [PATCH 4/5] hibernation, KFENCE: explicitly map/unmap KFENCE pages

["David Hildenbrand (Arm)" <david@kernel.org>]

On 9/17/26 08:07, Mike Rapoport (Microsoft) wrote:
> The pages protected by KFENCE are removed from the direct map.
> 
> safe_copy_page() temporarily maps and unmaps them using set_direct_map
> APIs, or, when the stars align, even using debug_pagealloc_map_pages().
> 
> Neither of these APIs cares whether it is a KFENCE page and both blindly
> perform the update of the kernel page table for any non-present page.

Agreed, we should route this through the actual mechanism that modified the
directmap in the first place.

> 
> Ability to use debug_pagealloc_map_pages() to remap KFENCE pages when both
> KFENCE and debug_pagealloc are enabled is an amusing coincidence.
> 
> But with increasing appetite for using set_direct_map for hardening
> purposes, it becomes too big of a hammer to enable saving any non-present
> page in the hibernation image.
> 
> Another gotcha is that loongarch that does not have a direct map at all
> advertises ARCH_HAS_SET_DIRECT_MAP to allow coexistence of KFENCE and
> hibernation.
> 
> Extend KFENCE with a bitmap that tracks which pages are protected and
> provide kfence_force_mapping() and kfence_restore_mapping() APIs that allow
> forced mapping and unmapping of KFENCE pages.
> 
> Use these APIs in hibernate_{map,unmap}_pages() for KFENCE pages.
> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  include/linux/kfence.h  | 29 +++++++++++++++++++++++++++
>  kernel/power/snapshot.c |  7 +++++++
>  mm/kfence/core.c        | 52 +++++++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 86 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kfence.h b/include/linux/kfence.h
> index e5822f6e7f279..33a126cb6d1b7 100644
> --- a/include/linux/kfence.h
> +++ b/include/linux/kfence.h
> @@ -222,6 +222,32 @@ struct kmem_obj_info;
>  bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab);
>  #endif
>  
> +/**
> + * kfence_force_mapping() - make sure a KFENCE page is mapped
> + * @page: page to map
> + *
> + * Check whether @page is protected and map it if needed.
> + *
> + * Requires: is_kfence_address(page_address(page))
> + *
> + * Return:
> + * * false - failed to map @page
> + * * true - @page is mapped
> + */
> +bool kfence_force_mapping(struct page *page);
> +
> +/**
> + * kfence_restore_mapping() - restore mapping of a KFENCE page


The name is misleading. You are actually restoring that the page is
unmapped/protected?

	kfence_force_mapping vs. kfence_restore_mapping

is confusing.

Can we find a better pair of function names that describe what is actually
happening?

Maybe something along the lines of

	kfence_prepare_copy_page

	kfence_finish_copy_page

That rather expresses what the caller intends to do.

-- 
Cheers,

David