Re: [PATCH v6 03/11] mm: introduce AS_NO_DIRECT_MAP

[Mike Rapoport <rppt@kernel.org>]

On Fri, Sep 12, 2025 at 09:17:34AM +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@amazon.co.uk>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  include/linux/pagemap.h   | 16 ++++++++++++++++
>  include/linux/secretmem.h | 18 ------------------
>  lib/buildid.c             |  4 ++--
>  mm/gup.c                  | 19 +++++--------------
>  mm/mlock.c                |  2 +-
>  mm/secretmem.c            |  8 ++------
>  6 files changed, 26 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 12a12dae727d..1f5739f6a9f5 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(const struct address_space *mapping)
> +{
> +	return test_bit(AS_NO_DIRECT_MAP, &mapping->flags);
> +}
> +
> +static inline bool vma_has_no_direct_map(const struct vm_area_struct *vma)
> +{
> +	return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
> +}
> +
>  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..89e567954284 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 folios without direct map entries (e.g. from 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..75a0cffdf37d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -11,7 +11,6 @@
>  #include <linux/rmap.h>
>  #include <linux/swap.h>
>  #include <linux/swapops.h>
> -#include <linux/secretmem.h>
>  
>  #include <linux/sched/signal.h>
>  #include <linux/rwsem.h>
> @@ -1234,7 +1233,7 @@ 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_has_no_direct_map(vma))
>  		return -EFAULT;
>  
>  	if (write) {
> @@ -2736,7 +2735,7 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
>   * This call assumes the caller has pinned the folio, that the lowest page table
>   * level still points to this folio, and that interrupts have been disabled.
>   *
> - * GUP-fast must reject all secretmem folios.
> + * GUP-fast must reject all folios without direct map entries (such as secretmem).
>   *
>   * Writing to pinned file-backed dirty tracked folios is inherently problematic
>   * (see comment describing the writable_file_mapping_allowed() function). We
> @@ -2751,7 +2750,6 @@ 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;
>  
>  	/*
> @@ -2763,18 +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;
> -
>  	if (WARN_ON_ONCE(folio_test_slab(folio)))
>  		return false;
>  
> -	/* hugetlb neither requires dirty-tracking nor can be secretmem. */
> +	/* hugetlb neither requires dirty-tracking nor can be without direct map. */
>  	if (folio_test_hugetlb(folio))
>  		return true;
>  
> @@ -2812,8 +2802,9 @@ 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..36f5e70faeb0 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_has_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..b5ce55079695 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,
> @@ -157,7 +152,7 @@ static void secretmem_free_folio(struct address_space *mapping,
>  	folio_zero_segment(folio, 0, folio_size(folio));
>  }
>  
> -const struct address_space_operations secretmem_aops = {
> +static const struct address_space_operations secretmem_aops = {
>  	.dirty_folio	= noop_dirty_folio,
>  	.free_folio	= secretmem_free_folio,
>  	.migrate_folio	= secretmem_migrate_folio,
> @@ -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
> 

-- 
Sincerely yours,
Mike.