Messages in this thread | | | Date | Tue, 4 Jul 2023 15:08:12 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH V2] arm64: properly define SOFT_DIRTY functionality |
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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@linux-foundation.org> > Cc: Anshuman Khandual <anshuman.khandual@arm.com> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: David Hildenbrand <david@redhat.com> > Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com> > Cc: Liu Shixin <liushixin2@huawei.com> > Cc: Will Deacon <will@kernel.org> > Cc: Yu Zhao <yuzhao@google.com> > Signed-off-by: Nico Pache <npache@redhat.com> > --- > 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 >
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