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

Nikita Kalyazin
2026-04-10T15:26:15Z

On 23/03/2026 18:43, Ackerley Tng wrote:
> "Kalyazin, Nikita" <kalyazin@amazon.co.uk> writes:
>
>> 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.
>>
>> 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;
>> +}
>> +
>> +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.
>> + *
>> + * 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;
>> +
>> + ret = set_direct_map_valid_noflush(addr, folio_nr_pages(folio), false);
>> + flush_tlb_kernel_range((unsigned long)addr,
>> + (unsigned long)addr + folio_size(folio));
>> +
>> + return ret;
>> +}
>> +EXPORT_SYMBOL_FOR_MODULES(folio_zap_direct_map, "kvm");
>> +
>> +/**
>> + * folio_restore_direct_map - restore the kernel direct map entry for a folio
>> + * @folio: folio whose direct map entry is to be restored
>> + *
>> + * This may only be called after a prior successful folio_zap_direct_map() on
>> + * the same folio. Because the zap will have already split any huge pages in
>> + * the direct map, restoration here only updates protection bits and cannot
>> + * fail.
>> + */
>> +void folio_restore_direct_map(struct folio *folio)
>> +{
>> + WARN_ON_ONCE(set_direct_map_valid_noflush(folio_address(folio),
>> + folio_nr_pages(folio), true));
>> +}
>> +EXPORT_SYMBOL_FOR_MODULES(folio_restore_direct_map, "kvm");
>> +#endif /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
>> --
>> 2.50.1
>
> Reviewed-by: Ackerley Tng <ackerleytng@google.com>
>
> I also took a look at Sashiko's [1] comments and I think that the
> highmem folio issues should be the responsibility of the caller to
> check.

Thank you.

>
> [1] https://sashiko.dev/#/patchset/20260317141031.514-1-kalyazin%40amazon.com