[PATCH] arm64: dirty memory check, enable soft dirty for arm64

[Steve Capper]

Hi Bin,

On Thu, Nov 30, 2017 at 12:14:09AM -0800, Bin Lu wrote:
>     arch/arm64/Kconfig: enable CONFIG_HAVE_ARCH_SOFT_DIRTY
>     arch/arm64/include/asm/pgtable.h: enable functions for soft dirty
> 
>     Checkpoint/Restore In Userspace(CRIU) needs the page's change
>     to be soft tracked. This allows to do a pre checkpoint and then dump
>     only touched pages.
> 
>     This is done by using a PTE bit (PTE_DIRTY) when
>     the page is backed in memory, and a new _PAGE_SWP_SOFT_DIRTY bit when
>     the page is swapped out.
> 
>     The bit of PTE_DIRTY is already defined in pgtable-prot.h.
> 
>     The _PAGE_SWP_SOFT_DIRTY bit is set as 59 in the swap pte.
> 
>     pmd_dirty and pmd_mkclean for THP page MADV_FREE also were
>     supported in this patch.
> 
>     This patch has been tested with ZDTM test suite in CRIU.
>     Such as: ./zdtm.py run -t zdtm/static/env00 --pre 10
> Signed-off-by: Bin Lu <bin.lu at arm.com>
> ---
>  arch/arm64/Kconfig               |  1 +
>  arch/arm64/include/asm/pgtable.h | 46 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index dfd9086..933f5be 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -73,6 +73,7 @@ config ARM64
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>  	select HAVE_ARCH_SECCOMP_FILTER
> +	select HAVE_ARCH_SOFT_DIRTY
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
>  	select HAVE_ARM_SMCCC
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 6eae342..8ab251e 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -306,6 +306,28 @@ static inline pgprot_t mk_sect_prot(pgprot_t prot)
>  	return __pgprot(pgprot_val(prot) & ~PTE_TABLE_BIT);
>  }
>  
> +#ifdef CONFIG_HAVE_ARCH_SOFT_DIRTY
> +static inline bool pte_soft_dirty(pte_t pte)
> +{
> +        return pte_sw_dirty(pte);
> +}
> +
> +static inline pte_t pte_mksoft_dirty(pte_t pte)
> +{
> +        return pte_mkdirty(pte);
> +}
> +
> +static inline pte_t pte_clear_soft_dirty(pte_t pte)
> +{
> +        return pte_mkclean(pte);
> +}
> +
> +#define pmd_soft_dirty(pmd)    pte_soft_dirty(pmd_pte(pmd))
> +#define pmd_mksoft_dirty(pmd)  pte_pmd(pte_mksoft_dirty(pmd_pte(pmd)))
> +#define pmd_clear_soft_dirty(pmd) pte_pmd(pte_clear_soft_dirty(pmd_pte(pmd)))
> +
> +#endif /* CONFIG_HAVE_ARCH_SOFT_DIRTY */
> +
>  #ifdef CONFIG_NUMA_BALANCING
>  /*
>   * See the comment in include/asm-generic/pgtable.h
> @@ -695,10 +717,12 @@ extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>   *	bits 2-7:	swap type
>   *	bits 8-57:	swap offset
>   *	bit  58:	PTE_PROT_NONE (must be zero)
> + *	bit  59:	swap software dirty tracking
>   */
>  #define __SWP_TYPE_SHIFT	2
>  #define __SWP_TYPE_BITS		6
>  #define __SWP_OFFSET_BITS	50
> +#define __SWP_PROT_NONE_BITS	1
>  #define __SWP_TYPE_MASK		((1 << __SWP_TYPE_BITS) - 1)
>  #define __SWP_OFFSET_SHIFT	(__SWP_TYPE_BITS + __SWP_TYPE_SHIFT)
>  #define __SWP_OFFSET_MASK	((1UL << __SWP_OFFSET_BITS) - 1)
> @@ -710,6 +734,28 @@ extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
>  #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })
>  #define __swp_entry_to_pte(swp)	((pte_t) { (swp).val })
>  
> +#ifdef CONFIG_MEM_SOFT_DIRTY
> +#define _PAGE_SWP_SOFT_DIRTY   (1UL << (__SWP_OFFSET_SHIFT + __SWP_OFFSET_BITS + __SWP_PROT_NONE_BITS))
> +#else
> +#define _PAGE_SWP_SOFT_DIRTY    0UL
> +#endif /* CONFIG_MEM_SOFT_DIRTY */
> +
> +#ifdef CONFIG_HAVE_ARCH_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 */
>  /*
>   * Ensure that there are not more swap files than can be encoded in the kernel
>   * PTEs.
> --

This looks good to me. Soft dirty pte's are created by both pte_mkdirty
and pte_mksoft_dirty. If a soft-dirty bit is cleared by
clear_soft_dirty(.), then the pte is also made read-only. Meaning
hardware DBM won't set the pte as dirty on write, a fault will take place
and then the pte will be made both writable and soft dirty.

Some changes to ptep_set_wrprotect(.) to preserve the dirty bit may
cause more soft dirty pte's to appear:
http://lists.infradead.org/pipermail/linux-arm-kernel/2017-December/545151.html

I *think* that shouldn't affect the use case I understand for the
patch - to identify whether or not a page has been written to since
last cleaned. 

FWIW:
Reviewed-by: Steve Capper <steve.capper at arm.com>

Cheers,
-- 
Steve