Subject: Re: [PATCH 1/2] arm64/mm: Change THP helpers to comply with generic MM semantics

From: Jonathan Cameron <[email-masked]>
Date: Tue, 18 Aug 2020 10:13:01 +0100

On Mon, 17 Aug 2020 14:49:43 +0530
Anshuman Khandual <[email-masked]> wrote:

> pmd_present() and pmd_trans_huge() are expected to behave in the following
> manner during various phases of a given PMD. It is derived from a previous
> detailed discussion on this topic [1] and present THP documentation [2].
> 
> pmd_present(pmd):
> 
> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd)
> 
> pmd_trans_huge(pmd):
> 
> - Returns true if pmd refers to system RAM and is a trans huge mapping
> 
> -------------------------------------------------------------------------
> |	PMD states	|	pmd_present	|	pmd_trans_huge	|
> -------------------------------------------------------------------------
> |	Mapped		|	Yes		|	Yes		|
> -------------------------------------------------------------------------
> |	Splitting	|	Yes		|	Yes		|
> -------------------------------------------------------------------------
> |	Migration/Swap	|	No		|	No		|
> -------------------------------------------------------------------------
> 
> The problem:
> 
> PMD is first invalidated with pmdp_invalidate() before it's splitting. This
> invalidation clears PMD_SECT_VALID as below.
> 
> PMD Split -> pmdp_invalidate() -> pmd_mkinvalid -> Clears PMD_SECT_VALID
> 
> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false
> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to re-
> affirm pmd_present() as true during the THP split process. To comply with
> above mentioned semantics, pmd_trans_huge() should also check pmd_present()
> first before testing presence of an actual transparent huge mapping.
> 
> The solution:
> 
> Ideally PMD_TYPE_SECT should have been used here instead. But it shares the
> bit position with PMD_SECT_VALID which is used for THP invalidation. Hence
> it will not be there for pmd_present() check after pmdp_invalidate().
> 
> A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD
> entry during invalidation which can help pmd_present() return true and in
> recognizing the fact that it still points to memory.
> 
> This bit is transient. During the split process it will be overridden by a
> page table page representing normal pages in place of erstwhile huge page.
> Other pmdp_invalidate() callers always write a fresh PMD value on the entry
> overriding this transient PMD_PRESENT_INVALID bit, which makes it safe.
> 
> [1]: https://lkml.org/lkml/2018/10/17/231
> [2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt

Hi Anshuman,

One query on this.  From my reading of the ARM ARM, bit 59 is not
an ignored bit.  The exact requirements for hardware to be using
it are a bit complex though.

It 'might' be safe to use it for this, but if so can we have a comment
explaining why.  Also more than possible I'm misunderstanding things! 

Thanks,

Jonathan

> 
> Cc: Catalin Marinas <[email-masked]>
> Cc: Will Deacon <[email-masked]>
> Cc: Mark Rutland <[email-masked]>
> Cc: Marc Zyngier <[email-masked]>
> Cc: Suzuki Poulose <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---
>  arch/arm64/include/asm/pgtable-prot.h |  7 ++++++
>  arch/arm64/include/asm/pgtable.h      | 34 ++++++++++++++++++++++++---
>  2 files changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
> index 4d867c6446c4..28792fdd9627 100644
> --- a/arch/arm64/include/asm/pgtable-prot.h
> +++ b/arch/arm64/include/asm/pgtable-prot.h
> @@ -19,6 +19,13 @@
>  #define PTE_DEVMAP		(_AT(pteval_t, 1) << 57)
>  #define PTE_PROT_NONE		(_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */
>  
> +/*
> + * This help indicate that the entry is present i.e pmd_page()
> + * still points to a valid huge page in memory even if the pmd
> + * has been invalidated.
> + */
> +#define PMD_PRESENT_INVALID	(_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
> +
>  #ifndef __ASSEMBLY__
>  
>  #include <asm/cpufeature.h>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index d5d3fbe73953..7aa69cace784 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -145,6 +145,18 @@ static inline pte_t set_pte_bit(pte_t pte, pgprot_t prot)
>  	return pte;
>  }
>  
> +static inline pmd_t clr_pmd_bit(pmd_t pmd, pgprot_t prot)
> +{
> +	pmd_val(pmd) &= ~pgprot_val(prot);
> +	return pmd;
> +}
> +
> +static inline pmd_t set_pmd_bit(pmd_t pmd, pgprot_t prot)
> +{
> +	pmd_val(pmd) |= pgprot_val(prot);
> +	return pmd;
> +}
> +
>  static inline pte_t pte_wrprotect(pte_t pte)
>  {
>  	pte = clear_pte_bit(pte, __pgprot(PTE_WRITE));
> @@ -363,15 +375,24 @@ static inline int pmd_protnone(pmd_t pmd)
>  }
>  #endif
>  
> +#define pmd_present_invalid(pmd)     (!!(pmd_val(pmd) & PMD_PRESENT_INVALID))
> +
> +static inline int pmd_present(pmd_t pmd)
> +{
> +	return pte_present(pmd_pte(pmd)) || pmd_present_invalid(pmd);
> +}
> +
>  /*
>   * THP definitions.
>   */
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -#define pmd_trans_huge(pmd)	(pmd_val(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))
> +static inline int pmd_trans_huge(pmd_t pmd)
> +{
> +	return pmd_val(pmd) && pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT);
> +}
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> -#define pmd_present(pmd)	pte_present(pmd_pte(pmd))
>  #define pmd_dirty(pmd)		pte_dirty(pmd_pte(pmd))
>  #define pmd_young(pmd)		pte_young(pmd_pte(pmd))
>  #define pmd_valid(pmd)		pte_valid(pmd_pte(pmd))
> @@ -381,7 +402,14 @@ static inline int pmd_protnone(pmd_t pmd)
>  #define pmd_mkclean(pmd)	pte_pmd(pte_mkclean(pmd_pte(pmd)))
>  #define pmd_mkdirty(pmd)	pte_pmd(pte_mkdirty(pmd_pte(pmd)))
>  #define pmd_mkyoung(pmd)	pte_pmd(pte_mkyoung(pmd_pte(pmd)))
> -#define pmd_mkinvalid(pmd)	(__pmd(pmd_val(pmd) & ~PMD_SECT_VALID))
> +
> +static inline pmd_t pmd_mkinvalid(pmd_t pmd)
> +{
> +	pmd = set_pmd_bit(pmd, __pgprot(PMD_PRESENT_INVALID));
> +	pmd = clr_pmd_bit(pmd, __pgprot(PMD_SECT_VALID));
> +
> +	return pmd;
> +}
>  
>  #define pmd_thp_or_huge(pmd)	(pmd_huge(pmd) || pmd_trans_huge(pmd))
>  

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