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.mari...@arm.com> Cc: Will Deacon <w...@kernel.org> Cc: Mark Rutland <mark.rutl...@arm.com> Cc: Marc Zyngier <m...@kernel.org> Cc: Suzuki Poulose <suzuki.poul...@arm.com> Cc: linux-arm-ker...@lists.infradead.org Cc: linux-kernel@vger.kernel.org Reviewed-by: Catalin Marinas <catalin.mari...@arm.com> Signed-off-by: Anshuman Khandual <anshuman.khand...@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
