Re: [PATCH v5 23/25] arm64/mm: Implement pte_batch_hint()
From: Mark Rutland <mark.rutland@arm.com>
Date: 2024-02-13 16:54:58
Also in:
linux-arm-kernel, linux-mm, lkml
On Fri, Feb 02, 2024 at 08:07:54AM +0000, Ryan Roberts wrote:
When core code iterates over a range of ptes and calls ptep_get() for each of them, if the range happens to cover contpte mappings, the number of pte reads becomes amplified by a factor of the number of PTEs in a contpte block. This is because for each call to ptep_get(), the implementation must read all of the ptes in the contpte block to which it belongs to gather the access and dirty bits. This causes a hotspot for fork(), as well as operations that unmap memory such as munmap(), exit and madvise(MADV_DONTNEED). Fortunately we can fix this by implementing pte_batch_hint() which allows their iterators to skip getting the contpte tail ptes when gathering the batch of ptes to operate on. This results in the number of PTE reads returning to 1 per pte. Tested-by: John Hubbard <jhubbard@nvidia.com> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com> Mark.
quoted hunk ↗ jump to hunk
--- arch/arm64/include/asm/pgtable.h | 9 +++++++++ 1 file changed, 9 insertions(+)diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index ad04adb7b87f..353ea67b5d75 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h@@ -1220,6 +1220,15 @@ static inline void contpte_try_unfold(struct mm_struct *mm, unsigned long addr, __contpte_try_unfold(mm, addr, ptep, pte); } +#define pte_batch_hint pte_batch_hint +static inline unsigned int pte_batch_hint(pte_t *ptep, pte_t pte) +{ + if (!pte_valid_cont(pte)) + return 1; + + return CONT_PTES - (((unsigned long)ptep >> 3) & (CONT_PTES - 1)); +} + /* * The below functions constitute the public API that arm64 presents to the * core-mm to manipulate PTE entries within their page tables (or at least this-- 2.25.1