On Thu, Aug 28, 2025 at 09:39:19AM +0000, Roy, Patrick wrote:
> 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).
>
> Signed-off-by: Patrick Roy <roypat@xxxxxxxxxxxx>
> ---
> include/linux/pagemap.h | 16 ++++++++++++++++
> include/linux/secretmem.h | 18 ------------------
> lib/buildid.c | 4 ++--
> mm/gup.c | 14 +++-----------
> mm/mlock.c | 2 +-
> mm/secretmem.c | 6 +-----
> 6 files changed, 23 insertions(+), 37 deletions(-)
>
> diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
> index e918f96881f5..0ae1fb057b3d 100644
> --- a/include/linux/secretmem.h
> +++ b/include/linux/secretmem.h
> @@ -4,28 +4,10 @@
>
> #ifdef CONFIG_SECRETMEM
>
> -extern const struct address_space_operations secretmem_aops;
Please also make secretmem_aops static in mm/secretmem.c
> -static inline bool secretmem_mapping(struct address_space *mapping)
> -{
> - return mapping->a_ops == &secretmem_aops;
> -}
> -
...
> diff --git a/mm/gup.c b/mm/gup.c
> index adffe663594d..8c988e076e5d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1234,7 +1234,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> if ((gup_flags & FOLL_SPLIT_PMD) && is_vm_hugetlb_page(vma))
> return -EOPNOTSUPP;
>
> - if (vma_is_secretmem(vma))
> + if (vma_is_no_direct_map(vma))
> return -EFAULT;
>
> if (write) {
> @@ -2751,7 +2751,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
> {
> bool reject_file_backed = false;
> struct address_space *mapping;
> - bool check_secretmem = false;
> unsigned long mapping_flags;
>
> /*
> @@ -2763,14 +2762,6 @@ 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;
> -
> if (WARN_ON_ONCE(folio_test_slab(folio)))
> return false;
There's a check for hugetlb after this and a comment there mentions
secretmem, please update that to "mapping with no direct map" or something
like that.
--
Sincerely yours,
Mike.
