linux-kernel - [PATCH] csky: mmu: Prevent spurious page faults

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Date:   Fri,  7 Apr 2023 04:54:34 -0400
From:   guoren@...nel.org
To:     guoren@...nel.org
Cc:     linux-csky@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, Guo Ren <guoren@...ux.alibaba.com>
Subject: [PATCH] csky: mmu: Prevent spurious page faults

From: Guo Ren <guoren@...ux.alibaba.com>

C-SKY MMU would pre-fetch invalid pte entries, and it could work with
flush_tlb_fix_spurious_fault, but the additional page fault exceptions
would reduce performance. So flushing the entry of the TLB would prevent
the following spurious page faults. Here is the test code:

define DATA_LEN  4096
define COPY_NUM  (504*100)

unsigned char src[DATA_LEN*COPY_NUM] = {0};
unsigned char dst[DATA_LEN*COPY_NUM] = {0};

unsigned char func_src[DATA_LEN*COPY_NUM] = {0};
unsigned char func_dst[DATA_LEN*COPY_NUM] = {0};

void main(void)
{
	int j;
	for (j = 0; j < COPY_NUM; j++)
		memcpy(&dst[j*DATA_LEN], &src[j*DATA_LEN], 4);
}

perf stat -e page-faults ./main.elf

The amount of page fault traps would be reduced in half with the patch.

Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/csky/abiv1/cacheflush.c | 2 ++
 arch/csky/abiv2/cacheflush.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/arch/csky/abiv1/cacheflush.c b/arch/csky/abiv1/cacheflush.c
index fb91b069dc69..6f38cc7944e1 100644
--- a/arch/csky/abiv1/cacheflush.c
+++ b/arch/csky/abiv1/cacheflush.c
@@ -40,6 +40,8 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long addr,
 	unsigned long pfn = pte_pfn(*ptep);
 	struct page *page;
 
+	flush_tlb_page(vma, addr);
+
 	if (!pfn_valid(pfn))
 		return;
 
diff --git a/arch/csky/abiv2/cacheflush.c b/arch/csky/abiv2/cacheflush.c
index 39c51399dd81..ff39c897c820 100644
--- a/arch/csky/abiv2/cacheflush.c
+++ b/arch/csky/abiv2/cacheflush.c
@@ -12,6 +12,8 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 	unsigned long addr;
 	struct page *page;
 
+	flush_tlb_page(vma, address);
+
 	if (!pfn_valid(pte_pfn(*pte)))
 		return;
 
-- 
2.36.1