[PATCH 0/2] arm64/mm: Enable THP migration

[PATCH 0/2] arm64/mm: Enable THP migration

[Anshuman Khandual]

This series enables THP migration on arm64 via ARCH_ENABLE_THP_MIGRATION.
But first this modifies all existing THP helpers like pmd_present() and
pmd_trans_huge() etc per expected generic memory semantics as concluded
from a previous discussion here.

https://lkml.org/lkml/2018/10/9/220

This series is based on v5.9-rc1.

Changes in V1:

- Used new PMD_PRESENT_INVALID (bit 59) to represent invalidated PMD state per Catalin

Changes in RFC V2: (https://patchwork.kernel.org/project/linux-mm/list/?series=302965)

- Used PMD_TABLE_BIT to represent splitting PMD state per Catalin

Changes in RFC V1: (https://patchwork.kernel.org/project/linux-mm/list/?series=138797)

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Anshuman Khandual (2):
  arm64/mm: Change THP helpers to comply with generic MM semantics
  arm64/mm: Enable THP migration

 arch/arm64/Kconfig                    |  4 +++
 arch/arm64/include/asm/pgtable-prot.h |  7 +++++
 arch/arm64/include/asm/pgtable.h      | 39 ++++++++++++++++++++++++---
 3 files changed, 47 insertions(+), 3 deletions(-)

-- 
2.20.1

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

[Anshuman Khandual]

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

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 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))
 
-- 
2.20.1

[PATCH 2/2] arm64/mm: Enable THP migration

[Anshuman Khandual]

In certain page migration situations, a THP page can be migrated without
being split into it's constituent subpages. This saves time required to
split a THP and put it back together when required. But it also saves an
wider address range translation covered by a single TLB entry, reducing
future page fault costs.

A previous patch changed platform THP helpers per generic memory semantics,
clearing the path for THP migration support. This adds two more THP helpers
required to create PMD migration swap entries. Now just enable HP migration
via ARCH_ENABLE_THP_MIGRATION.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/Kconfig               | 4 ++++
 arch/arm64/include/asm/pgtable.h | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 6d232837cbee..e21b94061780 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1876,6 +1876,10 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
 	def_bool y
 	depends on HUGETLB_PAGE && MIGRATION
 
+config ARCH_ENABLE_THP_MIGRATION
+	def_bool y
+	depends on TRANSPARENT_HUGEPAGE
+
 menu "Power management options"
 
 source "kernel/power/Kconfig"
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 7aa69cace784..c54334bca4e2 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -875,6 +875,11 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
 #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })
 #define __swp_entry_to_pte(swp)	((pte_t) { (swp).val })
 
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+#define __pmd_to_swp_entry(pmd)		((swp_entry_t) { pmd_val(pmd) })
+#define __swp_entry_to_pmd(swp)		__pmd((swp).val)
+#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
+
 /*
  * Ensure that there are not more swap files than can be encoded in the kernel
  * PTEs.
-- 
2.20.1

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

[Jonathan Cameron]

On Mon, 17 Aug 2020 14:49:43 +0530
Anshuman Khandual <anshuman.khandual@arm.com> 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 <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  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))
>  

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

[Anshuman Khandual]

On 08/18/2020 02:43 PM, Jonathan Cameron wrote:
> On Mon, 17 Aug 2020 14:49:43 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> 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! 

We are using this bit 59 only when the entry is not active from MMU
perspective i.e PMD_SECT_VALID is clear.

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

[Jonathan Cameron]

On Tue, 18 Aug 2020 15:11:58 +0530
Anshuman Khandual <anshuman.khandual@arm.com> wrote:

> On 08/18/2020 02:43 PM, Jonathan Cameron wrote:
> > On Mon, 17 Aug 2020 14:49:43 +0530
> > Anshuman Khandual <anshuman.khandual@arm.com> 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!   
> 
> We are using this bit 59 only when the entry is not active from MMU
> perspective i.e PMD_SECT_VALID is clear.
> 

Understood. I guess we ran out of bits that were always ignored so had
to start using ones that are ignored in this particular state.

Jonathan

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

[Anshuman Khandual]

On 08/18/2020 05:56 PM, Jonathan Cameron wrote:
> On Tue, 18 Aug 2020 15:11:58 +0530
> Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> On 08/18/2020 02:43 PM, Jonathan Cameron wrote:
>>> On Mon, 17 Aug 2020 14:49:43 +0530
>>> Anshuman Khandual <anshuman.khandual@arm.com> 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!   
>>
>> We are using this bit 59 only when the entry is not active from MMU
>> perspective i.e PMD_SECT_VALID is clear.
>>
> 
> Understood. I guess we ran out of bits that were always ignored so had
> to start using ones that are ignored in this particular state.

Right, there are no more available SW PTE bits.

#define PTE_DIRTY               (_AT(pteval_t, 1) << 55)
#define PTE_SPECIAL             (_AT(pteval_t, 1) << 56)
#define PTE_DEVMAP              (_AT(pteval_t, 1) << 57)
#define PTE_PROT_NONE           (_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */

Earlier I had proposed using PTE_SPECIAL at PMD level for this purpose.
But Catalin prefers these unused bits as the entry is anyway invalid
and which also leaves aside PTE_SPECIAL at mapped PMD for later use.
There is already one comment near PMD_PRESENT_INVALID definition which
explains this situation.

+/*
+ * 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 */

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

[Catalin Marinas]

On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual 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)

The second bullet doesn't make much sense. If you have a pmd mapping of
some I/O memory, pmd_present() still returns true (as does
pte_present()).

> 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()

Nit: add another . after i.e

> + * 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;
> +}

Could you use clear_pmd_bit (instead of clr) for consistency with
clear_pte_bit()?

It would be good if the mm folk can do a sanity check on the assumptions
about pmd_present/pmdp_invalidate/pmd_trans_huge.

The patch looks fine to me otherwise, feel free to add:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

-- 
Catalin

Re: [PATCH 2/2] arm64/mm: Enable THP migration

[Catalin Marinas]

On Mon, Aug 17, 2020 at 02:49:44PM +0530, Anshuman Khandual wrote:
> In certain page migration situations, a THP page can be migrated without
> being split into it's constituent subpages. This saves time required to
> split a THP and put it back together when required. But it also saves an
> wider address range translation covered by a single TLB entry, reducing
> future page fault costs.
> 
> A previous patch changed platform THP helpers per generic memory semantics,
> clearing the path for THP migration support. This adds two more THP helpers
> required to create PMD migration swap entries. Now just enable HP migration

s/HP/THP/

> via ARCH_ENABLE_THP_MIGRATION.
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
>  arch/arm64/Kconfig               | 4 ++++
>  arch/arm64/include/asm/pgtable.h | 5 +++++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6d232837cbee..e21b94061780 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -1876,6 +1876,10 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
>  	def_bool y
>  	depends on HUGETLB_PAGE && MIGRATION
>  
> +config ARCH_ENABLE_THP_MIGRATION
> +	def_bool y
> +	depends on TRANSPARENT_HUGEPAGE
> +
>  menu "Power management options"
>  
>  source "kernel/power/Kconfig"
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 7aa69cace784..c54334bca4e2 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -875,6 +875,11 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
>  #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })
>  #define __swp_entry_to_pte(swp)	((pte_t) { (swp).val })
>  
> +#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
> +#define __pmd_to_swp_entry(pmd)		((swp_entry_t) { pmd_val(pmd) })
> +#define __swp_entry_to_pmd(swp)		__pmd((swp).val)
> +#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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

[Ralph Campbell]

On 9/3/20 9:56 AM, Catalin Marinas wrote:
> On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual 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)
> 
> The second bullet doesn't make much sense. If you have a pmd mapping of
> some I/O memory, pmd_present() still returns true (as does
> pte_present()).
> 
>> 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()
> 
> Nit: add another . after i.e

Another nit: "This help indicate" => "This helper indicates"

Maybe I should look at the series more. :-)

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

[Anshuman Khandual]

On 09/03/2020 10:26 PM, Catalin Marinas wrote:
> On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual 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)
> 
> The second bullet doesn't make much sense. If you have a pmd mapping of
> some I/O memory, pmd_present() still returns true (as does
> pte_present()).

Derived this from an earlier discussion (https://lkml.org/lkml/2018/10/17/231)
but current representation here might not be accurate.

Would this be any better ?

pmd_present(pmd):

- Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
- Returns false if pmd refers to a migration or swap entry

> 
>> 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()
> 
> Nit: add another . after i.e

Will fix.

> 
>> + * 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;
>> +}
> 
> Could you use clear_pmd_bit (instead of clr) for consistency with
> clear_pte_bit()?

Sure, will do.

> 
> It would be good if the mm folk can do a sanity check on the assumptions
> about pmd_present/pmdp_invalidate/pmd_trans_huge.
> 
> The patch looks fine to me otherwise, feel free to add:
> 
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>

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

[Anshuman Khandual]

On 09/03/2020 11:01 PM, Ralph Campbell wrote:
> 
> On 9/3/20 9:56 AM, Catalin Marinas wrote:
>> On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual 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)
>>
>> The second bullet doesn't make much sense. If you have a pmd mapping of
>> some I/O memory, pmd_present() still returns true (as does
>> pte_present()).
>>
>>> 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()
>>
>> Nit: add another . after i.e
> 
> Another nit: "This help indicate" => "This helper indicates"
> 
> Maybe I should look at the series more. :-)

It is talking about the new PTE bit being used here not any
helper. Though the following replacement might be better.

s/This help indicate/This bit indicates/

/*
 * 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 */

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

[Catalin Marinas]

On Tue, Sep 08, 2020 at 03:48:08PM +0530, Anshuman Khandual wrote:
> On 09/03/2020 10:26 PM, Catalin Marinas wrote:
> > On Mon, Aug 17, 2020 at 02:49:43PM +0530, Anshuman Khandual 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)
> > 
> > The second bullet doesn't make much sense. If you have a pmd mapping of
> > some I/O memory, pmd_present() still returns true (as does
> > pte_present()).
> 
> Derived this from an earlier discussion (https://lkml.org/lkml/2018/10/17/231)
> but current representation here might not be accurate.
> 
> Would this be any better ?
> 
> pmd_present(pmd):
> 
> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd)
> - Returns false if pmd refers to a migration or swap entry

Yes, that's better

-- 
Catalin