[PATCH V2 0/2] arm64/mm: Enable THP migration

[PATCH V2 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-rc4.

Changes in V2:

- Renamed clr_pmd_bit() as clear_pmd_bit() per Catalin
- Updated in-code documentation per Catalin and Ralph
- Updated commit message in the first patch per Catalin
- Updated commit message in the second patch per Catalin
- Added tags from Catalin

Changes in V1: (https://patchwork.kernel.org/project/linux-mm/list/?series=333627)

- 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


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[PATCH V2 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 refers to a migration or swap entry

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
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
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..2df4b75fce3c 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 bit indicates 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..d8258ae8fce0 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 clear_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 = clear_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


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[PATCH V2 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 enable THP 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
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
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 d8258ae8fce0..bc68da9f5706 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


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Re: [PATCH V2 0/2] arm64/mm: Enable THP migration

[Will Deacon]

On Wed, 9 Sep 2020 10:23:01 +0530, Anshuman Khandual wrote:
> 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
> 
> [...]

Applied to arm64 (for-next/mm), thanks!

[1/2] arm64/mm: Change THP helpers to comply with generic MM semantics
      https://git.kernel.org/arm64/c/b65399f6111b
[2/2] arm64/mm: Enable THP migration
      https://git.kernel.org/arm64/c/53fa117bb33c

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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