Re: [PATCH 1/1] riscv/mm: fix soft-dirty migration PMDs being treated as present

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/4/26 05:03, Lance Yang wrote:
> RISC-V uses _PAGE_EXEC for swap soft-dirty tracking when
> CONFIG_MEM_SOFT_DIRTY is enabled and Svrsw60t59b is available. That's
> a problem for PMD migration entries, since pmd_present() also checks
> _PAGE_LEAF (R/W/X) to recognize THPs with _PAGE_PRESENT temporarily
> cleared during splitting.

I'm curious: why do we have to set leaf indications for non-present things? The
HW sure will ignore it, right?

Is this a sw problem? Who needs that?

> 
> When a soft-dirty THP is migrated, set_pmd_migration_entry() preserves
> soft-dirty with pmd_swp_mksoft_dirty(), setting the X bit in the
> migration PMD. Even with _PAGE_PRESENT clear, we end up treating a
> migration PMD as a present THP! The fault handler skips
> pmd_migration_entry_wait(), and a write fault can end up in
> do_huge_pmd_wp_page(), where pmd_page() decodes the migration entry
> as a mapped PFN.

That sounds bad.

> 
> Move the swap soft-dirty bit to bit 12 and start the swap offset at
> bit 13 when CONFIG_MEM_SOFT_DIRTY is enabled. This keeps R/W/X clear
> in migration PMDs and lets us keep the existing pmd_present() check
> for invalidated THPs. Leave the offset at bit 12 when soft-dirty
> tracking is disabled.

That reduces the effective swap size (and PFN we can store). Could that be a
problem?

> 
> Fixes: 2a3ebad4db63 ("riscv: mm: add soft-dirty page tracking support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
> I found this while reviewing Usama's PMD-level swap entries series [1]
> and comparing the swap flag bits with the PMD presence checks across
> architectures.
> 
> [1] https://lore.kernel.org/all/20261002095503.3585565-1-usama.arif@linux.dev/
> 
>  arch/riscv/include/asm/pgtable-bits.h |  6 +++---
>  arch/riscv/include/asm/pgtable.h      | 10 ++++++----
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h
> index d5a86b4df3ce6..d84b459e010c1 100644
> --- a/arch/riscv/include/asm/pgtable-bits.h
> +++ b/arch/riscv/include/asm/pgtable-bits.h
> @@ -27,12 +27,12 @@
>  	((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.
> + * Bit 12 is reserved for swap soft-dirty tracking. The swap offset
> + * starts at bit 13 when CONFIG_MEM_SOFT_DIRTY is enabled.
>   */
>  #define _PAGE_SWP_SOFT_DIRTY						\
>  	((riscv_has_extension_unlikely(RISCV_ISA_EXT_SVRSW60T59B)) ?	\
> -	 _PAGE_EXEC : 0)
> +	 (1UL << 12) : 0)
>  #else
>  #define _PAGE_SOFT_DIRTY	0
>  #define _PAGE_SWP_SOFT_DIRTY	0
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index b644db16bda94..acaf6d3b54d04 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -1178,18 +1178,20 @@ static inline pud_t pud_modify(pud_t pud, pgprot_t newprot)
>   *
>   * Format of swap PTE:
>   *	bit            0:	_PAGE_PRESENT (zero)
> - *	bit       1 to 2:	(zero)
> - *	bit            3:	_PAGE_SWP_SOFT_DIRTY
> + *	bit       1 to 3:	_PAGE_LEAF (zero)
>   *	bit            4:	_PAGE_SWP_UFFD
>   *	bit            5:	_PAGE_PROT_NONE (zero)
>   *	bit            6:	exclusive marker
>   *	bits      7 to 11:	swap type
> - *	bits 12 to XLEN-1:	swap offset
> + *	bit           12:	_PAGE_SWP_SOFT_DIRTY (CONFIG_MEM_SOFT_DIRTY)
> + *	bits 12/13 to XLEN-1:	swap offset (without/with CONFIG_MEM_SOFT_DIRTY)
>   */
>  #define __SWP_TYPE_SHIFT	7
>  #define __SWP_TYPE_BITS		5
>  #define __SWP_TYPE_MASK		((1UL << __SWP_TYPE_BITS) - 1)
> -#define __SWP_OFFSET_SHIFT	(__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
> +#define __SWP_OFFSET_SHIFT \
> +	(__SWP_TYPE_BITS + __SWP_TYPE_SHIFT + \
> +	 IS_ENABLED(CONFIG_MEM_SOFT_DIRTY))
>  
>  #define MAX_SWAPFILES_CHECK()	\
>  	BUILD_BUG_ON(MAX_SWAPFILES_SHIFT > __SWP_TYPE_BITS)


-- 
Cheers,

David