Re: [PATCH v5 03/12] mm: introduce AS_NO_DIRECT_MAP

[Fuad Tabba <tabba@google.com>]

Hi Patrick,

On Thu, 28 Aug 2025 at 10:39, Roy, Patrick <roypat@amazon.co.uk> 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@amazon.co.uk>
> ---
>  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/pagemap.h b/include/linux/pagemap.h
> index 12a12dae727d..b52b28ae4636 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -211,6 +211,7 @@ enum mapping_flags {
>                                    folio contents */
>         AS_INACCESSIBLE = 8,    /* Do not attempt direct R/W access to the mapping */
>         AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
> +       AS_NO_DIRECT_MAP = 10,  /* Folios in the mapping are not in the direct map */
>         /* Bits 16-25 are used for FOLIO_ORDER */
>         AS_FOLIO_ORDER_BITS = 5,
>         AS_FOLIO_ORDER_MIN = 16,
> @@ -346,6 +347,21 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(struct address_spac
>         return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>  }
>
> +static inline void mapping_set_no_direct_map(struct address_space *mapping)
> +{
> +       set_bit(AS_NO_DIRECT_MAP, &mapping->flags);
> +}
> +
> +static inline bool mapping_no_direct_map(struct address_space *mapping)
> +{
> +       return test_bit(AS_NO_DIRECT_MAP, &mapping->flags);
> +}
> +
> +static inline bool vma_is_no_direct_map(const struct vm_area_struct *vma)
> +{
> +       return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
> +}
> +

Any reason vma is const whereas mapping in the function that it calls
(defined above it) isn't?

Cheers,
/fuad

>  static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
>  {
>         return mapping->gfp_mask;
> 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;
> -
> -static inline bool secretmem_mapping(struct address_space *mapping)
> -{
> -       return mapping->a_ops == &secretmem_aops;
> -}
> -
> -bool vma_is_secretmem(struct vm_area_struct *vma);
>  bool secretmem_active(void);
>
>  #else
>
> -static inline bool vma_is_secretmem(struct vm_area_struct *vma)
> -{
> -       return false;
> -}
> -
> -static inline bool secretmem_mapping(struct address_space *mapping)
> -{
> -       return false;
> -}
> -
>  static inline bool secretmem_active(void)
>  {
>         return false;
> diff --git a/lib/buildid.c b/lib/buildid.c
> index c4b0f376fb34..33f173a607ad 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -65,8 +65,8 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
>
>         freader_put_folio(r);
>
> -       /* reject secretmem folios created with memfd_secret() */
> -       if (secretmem_mapping(r->file->f_mapping))
> +       /* reject secretmem folios created with memfd_secret() or guest_memfd() */
> +       if (mapping_no_direct_map(r->file->f_mapping))
>                 return -EFAULT;
>
>         r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);
> 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;
>
> @@ -2812,8 +2803,9 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>          * At this point, we know the mapping is non-null and points to an
>          * address_space object.
>          */
> -       if (check_secretmem && secretmem_mapping(mapping))
> +       if (mapping_no_direct_map(mapping))
>                 return false;
> +
>         /* The only remaining allowed file system is shmem. */
>         return !reject_file_backed || shmem_mapping(mapping);
>  }
> diff --git a/mm/mlock.c b/mm/mlock.c
> index a1d93ad33c6d..0def453fe881 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -474,7 +474,7 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
>
>         if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
>             is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
> -           vma_is_dax(vma) || vma_is_secretmem(vma) || (oldflags & VM_DROPPABLE))
> +           vma_is_dax(vma) || vma_is_no_direct_map(vma) || (oldflags & VM_DROPPABLE))
>                 /* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
>                 goto out;
>
> diff --git a/mm/secretmem.c b/mm/secretmem.c
> index 422dcaa32506..a2daee0e63a5 100644
> --- a/mm/secretmem.c
> +++ b/mm/secretmem.c
> @@ -134,11 +134,6 @@ static int secretmem_mmap_prepare(struct vm_area_desc *desc)
>         return 0;
>  }
>
> -bool vma_is_secretmem(struct vm_area_struct *vma)
> -{
> -       return vma->vm_ops == &secretmem_vm_ops;
> -}
> -
>  static const struct file_operations secretmem_fops = {
>         .release        = secretmem_release,
>         .mmap_prepare   = secretmem_mmap_prepare,
> @@ -206,6 +201,7 @@ static struct file *secretmem_file_create(unsigned long flags)
>
>         mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
>         mapping_set_unevictable(inode->i_mapping);
> +       mapping_set_no_direct_map(inode->i_mapping);
>
>         inode->i_op = &secretmem_iops;
>         inode->i_mapping->a_ops = &secretmem_aops;
> --
> 2.50.1
>