Re: [PATCH v11 02/16] set_memory: add folio_{zap, restore}_direct_map helpers

[Nikita Kalyazin <kalyazin@amazon.com>]

On 23/03/2026 17:51, David Hildenbrand (Arm) wrote:
> On 3/17/26 15:10, Kalyazin, Nikita wrote:
>> From: Nikita Kalyazin <kalyazin@amazon.com>
>>
>> Let's provide folio_{zap,restore}_direct_map helpers as preparation for
>> supporting removal of the direct map for guest_memfd folios.
>> In folio_zap_direct_map(), flush TLB to make sure the data is not
>> accessible.
>>
>> The new helpers need to be accessible to KVM on architectures that
>> support guest_memfd (x86 and arm64).
>>
>> Direct map removal gives guest_memfd the same protection that
>> memfd_secret does, such as hardening against Spectre-like attacks
>> through in-kernel gadgets.
> 
> Maybe mention that there might be a double TLB flush on some
> architectures, but that that is something to figure out later. Same
> behavior in secretmem code where this will be used next.

Added, thanks.

> 
>>
>> Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
>> ---
>>   include/linux/set_memory.h | 13 ++++++++++++
>>   mm/memory.c                | 42 ++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 55 insertions(+)
>>
>> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
>> index 1a2563f525fc..24caea2931f9 100644
>> --- a/include/linux/set_memory.h
>> +++ b/include/linux/set_memory.h
>> @@ -41,6 +41,15 @@ static inline int set_direct_map_valid_noflush(const void *addr,
>>        return 0;
>>   }
>>
>> +static inline int folio_zap_direct_map(struct folio *folio)
>> +{
>> +     return 0;
> 
> Should we return -ENOSYS here or similar?

I'm not very certain about it because set_direct_map_* return 0 in this 
case.  Do we want them to behave differently?

> 
>> +}
>> +
>> +static inline void folio_restore_direct_map(struct folio *folio)
>> +{
>> +}
>> +
>>   static inline bool kernel_page_present(struct page *page)
>>   {
>>        return true;
>> @@ -57,6 +66,10 @@ static inline bool can_set_direct_map(void)
>>   }
>>   #define can_set_direct_map can_set_direct_map
>>   #endif
>> +
>> +int folio_zap_direct_map(struct folio *folio);
>> +void folio_restore_direct_map(struct folio *folio);
>> +
>>   #endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>>
>>   #ifdef CONFIG_X86_64
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 07778814b4a8..cab6bb237fc0 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -78,6 +78,7 @@
>>   #include <linux/sched/sysctl.h>
>>   #include <linux/pgalloc.h>
>>   #include <linux/uaccess.h>
>> +#include <linux/set_memory.h>
>>
>>   #include <trace/events/kmem.h>
>>
>> @@ -7478,3 +7479,44 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma)
>>        if (is_vm_hugetlb_page(vma))
>>                hugetlb_vma_unlock_read(vma);
>>   }
>> +
>> +#ifdef CONFIG_ARCH_HAS_SET_DIRECT_MAP
>> +/**
>> + * folio_zap_direct_map - remove a folio from the kernel direct map
>> + * @folio: folio to remove from the direct map
>> + *
>> + * Removes the folio from the kernel direct map and flushes the TLB.  This may
>> + * require splitting huge pages in the direct map, which can fail due to memory
>> + * allocation.
> 
> Best to mention
> 
> "So far, only order-0 folios are supported." and then ...
> 
>> + *
>> + * Return: 0 on success, or a negative error code on failure.
>> + */
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +     const void *addr = folio_address(folio);
>> +     int ret;
>> +
> 
> if (folio_test_large(folio))
>          return -EINVAL;

Added, thanks.

> 
> 
> With that,
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thank you.

> 
> --
> Cheers,
> 
> David