Re: [PATCH v3 03/26] mm: introduce AS_NO_DIRECT_MAP

On Sun, Jul 26, 2026 at 10:22:36PM +0000, Brendan Jackman wrote:
> From: Patrick Roy <[email protected]>
> 
> 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.
> 
> 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 <[email protected]>
> Signed-off-by: Nikita Kalyazin <[email protected]>
> [Moved zapping to page cache; removed review tags]
> Signed-off-by: Brendan Jackman <[email protected]>
> ---
>  include/linux/secretmem.h  |  18 --------
>  mm/secretmem.c             |  46 +++-----------------

Lovely :)
For the secretmem bits

Reviewed-by: Mike Rapoport (Microsoft) <[email protected]>

>  8 files changed, 128 insertions(+), 87 deletions(-)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 4e8b2b29f6d3e..011f6e34859cc 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -15,6 +15,7 @@
>  #include <linux/bitops.h>
>  #include <linux/hardirq.h> /* for in_interrupt() */
>  #include <linux/hugetlb_inline.h>
> +#include <linux/set_memory.h>
>  
>  struct folio_batch;
>  
> @@ -210,6 +211,7 @@ enum mapping_flags {
>  	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
>  	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
>  				   account usage to user cgroups */
> +	AS_NO_DIRECT_MAP = 11,	/* 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,
> @@ -345,6 +347,9 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct addres
>  	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>  }
>  
> +static inline unsigned int
> +mapping_max_folio_order(const struct address_space *mapping);
> +
>  static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)
>  {
>  	return mapping->gfp_mask;
> @@ -366,6 +371,24 @@ static inline void mapping_set_gfp_mask(struct address_space *m, gfp_t mask)
>  	m->gfp_mask = mask;
>  }
>  
> +static inline void mapping_set_no_direct_map(struct address_space *mapping)
> +{
> +	WARN_ON(!can_set_direct_map());
> +	/* folio_zap_direct_map() doesn't support large folios. */
> +	WARN_ON(mapping_max_folio_order(mapping));

Can't we use VM_WARN_ON() here?

> +	set_bit(AS_NO_DIRECT_MAP, &mapping->flags);
> +}

...

> +#else
> +static inline int prep_add_unmapped_folio(struct address_space *mapping, struct folio *folio)
> +{
> +	VM_WARN_ON(mapping_no_direct_map(mapping));
> +	return 0;
> +}
> +
> +static inline void prep_remove_unmapped_folio(struct address_space *mapping,
> +					      struct folio *folio)
> +{
> +	VM_WARN_ON(mapping_no_direct_map(mapping));
> +}
> +
> +static inline void prep_remove_unmapped_batch(struct address_space *mapping,
> +					      struct folio_batch *fbatch)
> +{
> +	VM_WARN_ON(mapping_no_direct_map(mapping));
> +}

Heh, that's a lot of WARNings :)
But I can see why you want them :)

> +#endif

-- 
Sincerely yours,
Mike.