Re: [PATCH RFC v7 2/3] riscv: mm: Add soft-dirty page tracking support

[Chunyan Zhang <zhang.lyra@gmail.com>]

Hi Deepak,

On Sat, 7 Jun 2025 at 01:24, Deepak Gupta <debug@rivosinc.com> wrote:
>
> On Wed, Apr 09, 2025 at 05:53:19PM +0800, Chunyan Zhang wrote:
> >The Svrsw60t59b extension allows to free the PTE reserved bits 60 and 59
> >for software, this patch uses bit 59 for soft-dirty.
> >
> >To add swap PTE soft-dirty tracking, we borrow bit 3 which is available
> >for swap PTEs on RISC-V systems.
> >
> >Signed-off-by: Chunyan Zhang <zhangchunyan@iscas.ac.cn>
> >---
> > arch/riscv/Kconfig                    |  1 +
> > arch/riscv/include/asm/pgtable-bits.h | 19 +++++++
> > arch/riscv/include/asm/pgtable.h      | 71 ++++++++++++++++++++++++++-
> > 3 files changed, 89 insertions(+), 2 deletions(-)
> >
> >diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> >index 332fc00243ad..652e2bbfb702 100644
> >--- a/arch/riscv/Kconfig
> >+++ b/arch/riscv/Kconfig
> >@@ -139,6 +139,7 @@ config RISCV
> >       select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
> >       select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
> >       select HAVE_ARCH_SECCOMP_FILTER
> >+      select HAVE_ARCH_SOFT_DIRTY if 64BIT && MMU && RISCV_ISA_SVRSW60T59B
> >       select HAVE_ARCH_STACKLEAK
> >       select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> >       select HAVE_ARCH_TRACEHOOK
> >diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h
> >index a8f5205cea54..a6fa871dc19e 100644
> >--- a/arch/riscv/include/asm/pgtable-bits.h
> >+++ b/arch/riscv/include/asm/pgtable-bits.h
> >@@ -20,6 +20,25 @@
> >
> > #define _PAGE_SPECIAL   (1 << 8)    /* RSW: 0x1 */
> > #define _PAGE_DEVMAP    (1 << 9)    /* RSW, devmap */
> >+
> >+#ifdef CONFIG_MEM_SOFT_DIRTY
> >+
> >+/* ext_svrsw60t59b: bit 59 for software dirty tracking */
> >+#define _PAGE_SOFT_DIRTY                                              \
> >+      ((riscv_has_extension_unlikely(RISCV_ISA_EXT_SVRSW60T59B)) ?    \
> >+       (1UL << 59) : 0)
> >+/*
> >+ * Bit 3 is always zero for swap entry computation, so we
> >+ * can borrow it for swap page soft-dirty tracking.
> >+ */
> >+#define _PAGE_SWP_SOFT_DIRTY                                          \
> >+      ((riscv_has_extension_unlikely(RISCV_ISA_EXT_SVRSW60T59B)) ?    \
> >+       _PAGE_EXEC : 0)
> >+#else
> >+#define _PAGE_SOFT_DIRTY      0
> >+#define _PAGE_SWP_SOFT_DIRTY  0
> >+#endif /* CONFIG_MEM_SOFT_DIRTY */
> >+
>
> Above can be done like this
>
> +
> +#ifdef CONFIG_MEM_SOFT_DIRTY && RISCV_ISA_EXT_SVRSW60T59B
> +
> +/* ext_svrsw60t59b: bit 59 for software dirty tracking */
> +#define _PAGE_SOFT_DIRTY (1UL << 59)
> +/*
> + * Bit 3 is always zero for swap entry computation, so we
> + * can borrow it for swap page soft-dirty tracking.
> + */
> +#define _PAGE_SWP_SOFT_DIRTY _PAGE_EXEC
> +#else
> +#define _PAGE_SOFT_DIRTY       0
> +#define _PAGE_SWP_SOFT_DIRTY   0
> +#endif /* CONFIG_MEM_SOFT_DIRTY */
>

No, the feature depends not only on the compile-time configuration but
also on the run-time environment.

We need to check if the platform on which the system is running
supports the extension SVRSW60T59B, and riscv_has_extension_unlikely()
does that check that is a run-time check, not a build-time condition.

> > #define _PAGE_TABLE     _PAGE_PRESENT
> >
> > /*
> >diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> >index 428e48e5f57d..14461ffe6321 100644
> >--- a/arch/riscv/include/asm/pgtable.h
> >+++ b/arch/riscv/include/asm/pgtable.h
> >@@ -436,7 +436,7 @@ static inline pte_t pte_mkwrite_novma(pte_t pte)
>
> Shouldn't "static inline int pte_dirty(pte_t pte)" be updated as well

It may not be needed, since pte_dirty() on X86 and ARM doesn't check
_PAGE_SOFT_DIRTY either.

Thanks for the review,
Chunyan




>
> static inline int pte_dirty(pte_t pte)
> {
>         return pte_val(pte) & (_PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> }
>
> Perhaps have a macro which includes both dirty together and then use together.
>
>
> >
> > static inline pte_t pte_mkdirty(pte_t pte)
> > {
> >-      return __pte(pte_val(pte) | _PAGE_DIRTY);
> >+      return __pte(pte_val(pte) | _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
> > }
> >
> > static inline pte_t pte_mkclean(pte_t pte)
> >@@ -469,6 +469,38 @@ static inline pte_t pte_mkhuge(pte_t pte)
> >       return pte;
> > }
> >
> >+#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> >+static inline bool pte_soft_dirty(pte_t pte)
> >+{
> >+      return !!(pte_val(pte) & _PAGE_SOFT_DIRTY);
> >+}
> >+
> >+static inline pte_t pte_mksoft_dirty(pte_t pte)
> >+{
> >+      return __pte(pte_val(pte) | _PAGE_SOFT_DIRTY);
> >+}
> >+
> >+static inline pte_t pte_clear_soft_dirty(pte_t pte)
> >+{
> >+      return __pte(pte_val(pte) & ~(_PAGE_SOFT_DIRTY));
> >+}
> >+
> >+static inline bool pte_swp_soft_dirty(pte_t pte)
> >+{
> >+      return !!(pte_val(pte) & _PAGE_SWP_SOFT_DIRTY);
> >+}
> >+
> >+static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> >+{
> >+      return __pte(pte_val(pte) | _PAGE_SWP_SOFT_DIRTY);
> >+}
> >+
> >+static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> >+{
> >+      return __pte(pte_val(pte) & ~(_PAGE_SWP_SOFT_DIRTY));
> >+}
> >+#endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */
> >+
> > #ifdef CONFIG_RISCV_ISA_SVNAPOT
> > #define pte_leaf_size(pte)    (pte_napot(pte) ?                               \
> >                                       napot_cont_size(napot_cont_order(pte)) :\
> >@@ -821,6 +853,40 @@ static inline pud_t pud_mkspecial(pud_t pud)
> > }
> > #endif
> >
> >+#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> >+static inline bool pmd_soft_dirty(pmd_t pmd)
> >+{
> >+      return pte_soft_dirty(pmd_pte(pmd));
> >+}
> >+
> >+static inline pmd_t pmd_mksoft_dirty(pmd_t pmd)
> >+{
> >+      return pte_pmd(pte_mksoft_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)));
> >+}
> >+
> >+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> >+static inline bool pmd_swp_soft_dirty(pmd_t pmd)
> >+{
> >+      return pte_swp_soft_dirty(pmd_pte(pmd));
> >+}
> >+
> >+static inline pmd_t pmd_swp_mksoft_dirty(pmd_t pmd)
> >+{
> >+      return pte_pmd(pte_swp_mksoft_dirty(pmd_pte(pmd)));
> >+}
> >+
> >+static inline pmd_t pmd_swp_clear_soft_dirty(pmd_t pmd)
> >+{
> >+      return pte_pmd(pte_swp_clear_soft_dirty(pmd_pte(pmd)));
> >+}
> >+#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
> >+#endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */
> >+
> > static inline void set_pmd_at(struct mm_struct *mm, unsigned long addr,
> >                               pmd_t *pmdp, pmd_t pmd)
> > {
> >@@ -910,7 +976,8 @@ extern pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
> >  *
> >  * Format of swap PTE:
> >  *    bit            0:       _PAGE_PRESENT (zero)
> >- *    bit       1 to 3:       _PAGE_LEAF (zero)
> >+ *    bit       1 to 2:       (zero)
> >+ *    bit            3:       _PAGE_SWP_SOFT_DIRTY
> >  *    bit            5:       _PAGE_PROT_NONE (zero)
> >  *    bit            6:       exclusive marker
> >  *    bits      7 to 11:      swap type
> >--
> >2.34.1
> >

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