Re: [PATCH v9 01/13] set_memory: add folio_{zap, restore}_direct_map helpers

[Nikita Kalyazin <kalyazin@amazon.com>]

On 15/01/2026 10:54, Huacai Chen wrote:
> Hi, Nikita,

Hi Huacai,

> 
> On Wed, Jan 14, 2026 at 9:45 PM Kalyazin, Nikita <kalyazin@amazon.co.uk> wrote:
>>
>> From: Nikita Kalyazin <kalyazin@amazon.com>
>>
>> These allow guest_memfd to remove its memory from the direct map.
>> Only implement them for architectures that have direct map.
>> In folio_zap_direct_map(), flush TLB on architectures where
>> set_direct_map_valid_noflush() does not flush it internally.
>>
>> The new helpers need to be accessible to KVM on architectures that
>> support guest_memfd (x86 and arm64).  Since arm64 does not support
>> building KVM as a module, only export them on x86.
>>
>> 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>
>> ---
>>   arch/arm64/include/asm/set_memory.h     |  2 ++
>>   arch/arm64/mm/pageattr.c                | 12 ++++++++++++
>>   arch/loongarch/include/asm/set_memory.h |  2 ++
>>   arch/loongarch/mm/pageattr.c            | 16 ++++++++++++++++
>>   arch/riscv/include/asm/set_memory.h     |  2 ++
>>   arch/riscv/mm/pageattr.c                | 16 ++++++++++++++++
>>   arch/s390/include/asm/set_memory.h      |  2 ++
>>   arch/s390/mm/pageattr.c                 | 18 ++++++++++++++++++
>>   arch/x86/include/asm/set_memory.h       |  2 ++
>>   arch/x86/mm/pat/set_memory.c            | 20 ++++++++++++++++++++
>>   include/linux/set_memory.h              | 10 ++++++++++
>>   11 files changed, 102 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/set_memory.h b/arch/arm64/include/asm/set_memory.h
>> index 90f61b17275e..d949f1deb701 100644
>> --- a/arch/arm64/include/asm/set_memory.h
>> +++ b/arch/arm64/include/asm/set_memory.h
>> @@ -14,6 +14,8 @@ int set_memory_valid(unsigned long addr, int numpages, int enable);
>>   int set_direct_map_invalid_noflush(struct page *page);
>>   int set_direct_map_default_noflush(struct page *page);
>>   int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
>> +int folio_zap_direct_map(struct folio *folio);
>> +int folio_restore_direct_map(struct folio *folio);
>>   bool kernel_page_present(struct page *page);
>>
>>   int set_memory_encrypted(unsigned long addr, int numpages);
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index f0e784b963e6..a94eff324dda 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -357,6 +357,18 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>>          return set_memory_valid(addr, nr, valid);
>>   }
>>
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), false);
>> +}
>> +
>> +int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), true);
>> +}
>> +
>>   #ifdef CONFIG_DEBUG_PAGEALLOC
>>   /*
>>    * This is - apart from the return value - doing the same
>> diff --git a/arch/loongarch/include/asm/set_memory.h b/arch/loongarch/include/asm/set_memory.h
>> index 55dfaefd02c8..9bc80ac420a9 100644
>> --- a/arch/loongarch/include/asm/set_memory.h
>> +++ b/arch/loongarch/include/asm/set_memory.h
>> @@ -18,5 +18,7 @@ bool kernel_page_present(struct page *page);
>>   int set_direct_map_default_noflush(struct page *page);
>>   int set_direct_map_invalid_noflush(struct page *page);
>>   int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
>> +int folio_zap_direct_map(struct folio *folio);
>> +int folio_restore_direct_map(struct folio *folio);
>>
>>   #endif /* _ASM_LOONGARCH_SET_MEMORY_H */
>> diff --git a/arch/loongarch/mm/pageattr.c b/arch/loongarch/mm/pageattr.c
>> index f5e910b68229..14bd322dd112 100644
>> --- a/arch/loongarch/mm/pageattr.c
>> +++ b/arch/loongarch/mm/pageattr.c
>> @@ -236,3 +236,19 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>>
>>          return __set_memory(addr, 1, set, clear);
>>   }
>> +
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       int ret;
>> +
>> +       ret = set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                          folio_nr_pages(folio), false);
>> +
>> +       return ret;
> Why not use a single statement which is the same as the ARM64 version?
> The RISCV version has the same problem.

No reason for them to be different.  Will update in the next version. 
Thank you!

> 
> Huacai
> 
>> +}
>> +
>> +int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), true);
>> +}
>> diff --git a/arch/riscv/include/asm/set_memory.h b/arch/riscv/include/asm/set_memory.h
>> index 87389e93325a..16557b70c830 100644
>> --- a/arch/riscv/include/asm/set_memory.h
>> +++ b/arch/riscv/include/asm/set_memory.h
>> @@ -43,6 +43,8 @@ static inline int set_kernel_memory(char *startp, char *endp,
>>   int set_direct_map_invalid_noflush(struct page *page);
>>   int set_direct_map_default_noflush(struct page *page);
>>   int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
>> +int folio_zap_direct_map(struct folio *folio);
>> +int folio_restore_direct_map(struct folio *folio);
>>   bool kernel_page_present(struct page *page);
>>
>>   #endif /* __ASSEMBLER__ */
>> diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
>> index 3f76db3d2769..2c218868114b 100644
>> --- a/arch/riscv/mm/pageattr.c
>> +++ b/arch/riscv/mm/pageattr.c
>> @@ -401,6 +401,22 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>>          return __set_memory((unsigned long)page_address(page), nr, set, clear);
>>   }
>>
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       int ret;
>> +
>> +       ret = set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                          folio_nr_pages(folio), false);
>> +
>> +       return ret;
>> +}
>> +
>> +int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), true);
>> +}
>> +
>>   #ifdef CONFIG_DEBUG_PAGEALLOC
>>   static int debug_pagealloc_set_page(pte_t *pte, unsigned long addr, void *data)
>>   {
>> diff --git a/arch/s390/include/asm/set_memory.h b/arch/s390/include/asm/set_memory.h
>> index 94092f4ae764..fc73652e5715 100644
>> --- a/arch/s390/include/asm/set_memory.h
>> +++ b/arch/s390/include/asm/set_memory.h
>> @@ -63,6 +63,8 @@ __SET_MEMORY_FUNC(set_memory_4k, SET_MEMORY_4K)
>>   int set_direct_map_invalid_noflush(struct page *page);
>>   int set_direct_map_default_noflush(struct page *page);
>>   int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
>> +int folio_zap_direct_map(struct folio *folio);
>> +int folio_restore_direct_map(struct folio *folio);
>>   bool kernel_page_present(struct page *page);
>>
>>   #endif
>> diff --git a/arch/s390/mm/pageattr.c b/arch/s390/mm/pageattr.c
>> index d3ce04a4b248..df4a487b484d 100644
>> --- a/arch/s390/mm/pageattr.c
>> +++ b/arch/s390/mm/pageattr.c
>> @@ -412,6 +412,24 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>>          return __set_memory((unsigned long)page_to_virt(page), nr, flags);
>>   }
>>
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       unsigned long addr = (unsigned long)folio_address(folio);
>> +       int ret;
>> +
>> +       ret = set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                          folio_nr_pages(folio), false);
>> +       flush_tlb_kernel_range(addr, addr + folio_size(folio));
>> +
>> +       return ret;
>> +}
>> +
>> +int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), true);
>> +}
>> +
>>   bool kernel_page_present(struct page *page)
>>   {
>>          unsigned long addr;
>> diff --git a/arch/x86/include/asm/set_memory.h b/arch/x86/include/asm/set_memory.h
>> index 61f56cdaccb5..7208af609121 100644
>> --- a/arch/x86/include/asm/set_memory.h
>> +++ b/arch/x86/include/asm/set_memory.h
>> @@ -90,6 +90,8 @@ int set_pages_rw(struct page *page, int numpages);
>>   int set_direct_map_invalid_noflush(struct page *page);
>>   int set_direct_map_default_noflush(struct page *page);
>>   int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
>> +int folio_zap_direct_map(struct folio *folio);
>> +int folio_restore_direct_map(struct folio *folio);
>>   bool kernel_page_present(struct page *page);
>>
>>   extern int kernel_set_to_readonly;
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index 6c6eb486f7a6..3f0fc30eb320 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -2656,6 +2656,26 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
>>          return __set_pages_np(page, nr);
>>   }
>>
>> +int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       unsigned long addr = (unsigned long)folio_address(folio);
>> +       int ret;
>> +
>> +       ret = set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                          folio_nr_pages(folio), false);
>> +       flush_tlb_kernel_range(addr, addr + folio_size(folio));
>> +
>> +       return ret;
>> +}
>> +EXPORT_SYMBOL_FOR_MODULES(folio_zap_direct_map, "kvm");
>> +
>> +int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return set_direct_map_valid_noflush(folio_page(folio, 0),
>> +                                           folio_nr_pages(folio), true);
>> +}
>> +EXPORT_SYMBOL_FOR_MODULES(folio_restore_direct_map, "kvm");
>> +
>>   #ifdef CONFIG_DEBUG_PAGEALLOC
>>   void __kernel_map_pages(struct page *page, int numpages, int enable)
>>   {
>> diff --git a/include/linux/set_memory.h b/include/linux/set_memory.h
>> index 3030d9245f5a..8d1c8a7f7d79 100644
>> --- a/include/linux/set_memory.h
>> +++ b/include/linux/set_memory.h
>> @@ -40,6 +40,16 @@ static inline int set_direct_map_valid_noflush(struct page *page,
>>          return 0;
>>   }
>>
>> +static inline int folio_zap_direct_map(struct folio *folio)
>> +{
>> +       return 0;
>> +}
>> +
>> +static inline int folio_restore_direct_map(struct folio *folio)
>> +{
>> +       return 0;
>> +}
>> +
>>   static inline bool kernel_page_present(struct page *page)
>>   {
>>          return true;
>> --
>> 2.50.1
>>
>>