Re: [PATCH v7 03/12] mm: introduce AS_NO_DIRECT_MAP

David Hildenbrand
2025-09-25T10:25:40Z

On 24.09.25 17:10, Patrick Roy wrote:
> From: Patrick Roy <roypat@amazon.co.uk>
>
> Add AS_NO_DIRECT_MAP for mappings where direct map entries of folios are
> set to not present . Currently, mappings that match this description are
> secretmem mappings (memfd_secret()). Later, some guest_memfd
> configurations will also fall into this category.
>
> Reject this new type of mappings in all locations that currently reject
> secretmem mappings, on the assumption that if secretmem mappings are
> rejected somewhere, it is precisely because of an inability to deal with
> folios without direct map entries, and then make memfd_secret() use
> AS_NO_DIRECT_MAP on its address_space to drop its special
> vma_is_secretmem()/secretmem_mapping() checks.
>
> This drops a optimization in gup_fast_folio_allowed() where
> secretmem_mapping() was only called if CONFIG_SECRETMEM=y. secretmem is
> enabled by default since commit b758fe6df50d ("mm/secretmem: make it on
> by default"), so the secretmem check did not actually end up elided in
> most cases anymore anyway.
>
> Use a new flag instead of overloading AS_INACCESSIBLE (which is already
> set by guest_memfd) because not all guest_memfd mappings will end up
> being direct map removed (e.g. in pKVM setups, parts of guest_memfd that
> can be mapped to userspace should also be GUP-able, and generally not
> have restrictions on who can access it).
>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> ---

I enjoy seeing secretmem special-casing in common code go away.

[...]

>
> /*
> @@ -2763,18 +2761,10 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
> reject_file_backed = true;
>
> /* We hold a folio reference, so we can safely access folio fields. */
> -
> - /* secretmem folios are always order-0 folios. */
> - if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
> - check_secretmem = true;
> -
> - if (!reject_file_backed && !check_secretmem)
> - return true;
> -

Losing that optimization is not too bad I guess.

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers

David / dhildenb