Re: [PATCH V2] arm64: properly define SOFT_DIRTY functionality

[Mark Rutland]

On Tue, Jul 04, 2023 at 09:36:33AM -0400, Nico Pache wrote:
> ARM64 has a soft-dirty bit (software dirty) but never properly defines
> CONFIG_ARCH_HAS_SOFT_DIRTY or its necessary functions. This patch
> introduces the ability to set/clear the soft dirty bit in a similar
> manner as the other arches that utilize it.

Anshuman already explained that this is not correct -- to enable
CONFIG_ARCH_HAS_SOFT_DIRTY, you need *another* PTE bit. Please don't send
another version following this approach.

Despite its name, pte_sw_dirty() has nothing to do with
CONFIG_ARCH_HAS_SOFT_DIRTY. We have pte_hw_dirty() and pte_sw_dirty() because
with Hardware Dirty bit management the HW dirty bit is *also* the write
permission bit, and to have a dirty non-writeable PTE state we have to use a SW
bit, which is what pte_sw_dirty() handles. Both pte_hw_dirty() and
pte_sw_dirty() comprise the regular dirty state.

That's *very* different from CONFIG_ARCH_HAS_SOFT_DIRTY, which is about having
a *separate* software dirty state that can be used for longer-term dirty
tracking (whether the page was last touched since some management SW
manipulated the page).

> However, we must be careful... there are cases where the DBM bit is not
> available and the software dirty bit plays a essential role in determining
> whether or not a page is dirty. In these cases we must not allow the
> user to clear the software dirty bit. We can check for these cases by
> utilizing the arch_has_hw_pte_young() function which tests the availability
> of DBM.

Regardless of the above, this doesn't seem to have been thought through. why
would it be ok for this to work or not work dependent on DBM?

Thanks,
Mark.

> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
> Cc: Liu Shixin <liushixin2@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig               |   1 +
>  arch/arm64/include/asm/pgtable.h | 104 ++++++++++++++++++++++++++-----
>  2 files changed, 90 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 7856c3a3e35a..6ea73b8148c5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -173,6 +173,7 @@ config ARM64
>  	select HAVE_ARCH_PREL32_RELOCATIONS
>  	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	select HAVE_ARCH_SECCOMP_FILTER
> +	select HAVE_ARCH_SOFT_DIRTY
>  	select HAVE_ARCH_STACKLEAK
>  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
>  	select HAVE_ARCH_TRACEHOOK
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 0bd18de9fd97..c4970c9ed114 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -51,6 +51,20 @@ static inline bool arch_thp_swp_supported(void)
>  }
>  #define arch_thp_swp_supported arch_thp_swp_supported
>  
> +/*
> + * On arm64 without hardware Access Flag, copying from user will fail because
> + * the pte is old and cannot be marked young. So we always end up with zeroed
> + * page after fork() + CoW for pfn mappings. We don't always have a
> + * hardware-managed access flag on arm64.
> + */
> +#define arch_has_hw_pte_young		cpu_has_hw_af
> +
> +/*
> + * Experimentally, it's cheap to set the access flag in hardware and we
> + * benefit from prefaulting mappings as 'old' to start with.
> + */
> +#define arch_wants_old_prefaulted_pte	cpu_has_hw_af
> +
>  /*
>   * Outside of a few very special situations (e.g. hibernation), we always
>   * use broadcast TLB invalidation instructions, therefore a spurious page
> @@ -121,8 +135,9 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
>  })
>  
>  #define pte_hw_dirty(pte)	(pte_write(pte) && !(pte_val(pte) & PTE_RDONLY))
> -#define pte_sw_dirty(pte)	(!!(pte_val(pte) & PTE_DIRTY))
> -#define pte_dirty(pte)		(pte_sw_dirty(pte) || pte_hw_dirty(pte))
> +#define pte_soft_dirty(pte)	(!!(pte_val(pte) & PTE_DIRTY))
> +#define pte_dirty(pte)		(pte_soft_dirty(pte) || pte_hw_dirty(pte))
> +#define pte_swp_soft_dirty(pte)	pte_soft_dirty(pte)
>  
>  #define pte_valid(pte)		(!!(pte_val(pte) & PTE_VALID))
>  /*
> @@ -189,7 +204,8 @@ static inline pte_t pte_mkwrite(pte_t pte)
>  
>  static inline pte_t pte_mkclean(pte_t pte)
>  {
> -	pte = clear_pte_bit(pte, __pgprot(PTE_DIRTY));
> +	if (!arch_has_hw_pte_young())
> +		pte = clear_pte_bit(pte, __pgprot(PTE_DIRTY));
>  	pte = set_pte_bit(pte, __pgprot(PTE_RDONLY));
>  
>  	return pte;
> @@ -1077,25 +1093,83 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
>  #define phys_to_ttbr(addr)	(addr)
>  #endif
>  
> -/*
> - * On arm64 without hardware Access Flag, copying from user will fail because
> - * the pte is old and cannot be marked young. So we always end up with zeroed
> - * page after fork() + CoW for pfn mappings. We don't always have a
> - * hardware-managed access flag on arm64.
> - */
> -#define arch_has_hw_pte_young		cpu_has_hw_af
> +static inline bool pud_sect_supported(void)
> +{
> +	return PAGE_SIZE == SZ_4K;
> +}
>  
> +#ifdef CONFIG_ARM64_HW_AFDBM
>  /*
> - * Experimentally, it's cheap to set the access flag in hardware and we
> - * benefit from prefaulting mappings as 'old' to start with.
> + * if we have the DBM bit we can utilize the software dirty bit as
> + * a mechanism to introduce the soft_dirty functionality; however, without
> + * it this bit is crucial to determining if a entry is dirty and we cannot
> + * clear it via software. DBM can also be disabled or broken on some early
> + * armv8 devices, so check its availability before modifying it.
>   */
> -#define arch_wants_old_prefaulted_pte	cpu_has_hw_af
> +static inline pte_t pte_clear_soft_dirty(pte_t pte)
> +{
> +	if (!arch_has_hw_pte_young())
> +		return pte;
>  
> -static inline bool pud_sect_supported(void)
> +	return clear_pte_bit(pte, __pgprot(PTE_DIRTY));
> +}
> +
> +static inline pte_t pte_mksoft_dirty(pte_t pte)
>  {
> -	return PAGE_SIZE == SZ_4K;
> +	if (!arch_has_hw_pte_young())
> +		return pte;
> +
> +	return set_pte_bit(pte, __pgprot(PTE_DIRTY));
> +}
> +
> +static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> +{
> +	if (!arch_has_hw_pte_young())
> +		return pte;
> +
> +	return clear_pte_bit(pte, __pgprot(PTE_DIRTY));
> +}
> +
> +static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> +{
> +	if (!arch_has_hw_pte_young())
> +		return pte;
> +
> +	return set_pte_bit(pte, __pgprot(PTE_DIRTY));
> +}
> +
> +static inline int pmd_soft_dirty(pmd_t pmd)
> +{
> +	return pte_soft_dirty(pmd_pte(pmd));
> +}
> +
> +static inline pmd_t pmd_clear_soft_dirty(pmd_t pmd)
> +{
> +	return pte_pmd(pte_clear_soft_dirty(pmd_pte(pmd)));
> +}
> +
> +static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
> +{
> +	return pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)));
>  }
>  
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +static inline int pmd_swp_soft_dirty(pmd_t pmd)
> +{
> +	return pmd_soft_dirty(pmd);
> +}
> +
> +static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t pmd)
> +{
> +	return pmd_clear_soft_dirty(pmd);
> +}
> +
> +static inline pmd_t pmd_swp_mksoft_dirty(pmd_t pmd)
> +{
> +	return pmd_mksoft_dirty(pmd);
> +}
> +#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
> +#endif /* CONFIG_ARM64_HW_AFDBM */
>  
>  #define __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION
>  #define ptep_modify_prot_start ptep_modify_prot_start
> -- 
> 2.41.0
>