Thread (18 messages) 18 messages, 4 authors, 2023-08-26

Re: [PATCH rfc v2 05/10] powerpc: mm: use try_vma_locked_page_fault()

From: Christophe Leroy <hidden>
Date: 2023-08-22 09:38:59
Also in: linux-arm-kernel, linux-mm, linux-riscv, linux-s390, lkml, loongarch


Le 21/08/2023 à 14:30, Kefeng Wang a écrit :
Use new try_vma_locked_page_fault() helper to simplify code.
No functional change intended.
Does it really simplifies code ? It's 32 insertions versus 34 deletions 
so only removing 2 lines.

I don't like the struct vm_fault you are adding because when it was four 
independant variables it was handled through local registers. Now that 
it is a struct it has to go via the stack, leading to unnecessary memory 
read and writes. And going back and forth between architecture code and 
generic code may also be counter-performant.

Did you make any performance analysis ? Page faults are really a hot 
path when dealling with minor faults.

Thanks
Christophe
quoted hunk ↗ jump to hunk
Signed-off-by: Kefeng Wang <redacted>
---
  arch/powerpc/mm/fault.c | 66 ++++++++++++++++++++---------------------
  1 file changed, 32 insertions(+), 34 deletions(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index b1723094d464..52f9546e020e 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -391,6 +391,22 @@ static int page_fault_is_bad(unsigned long err)
  #define page_fault_is_bad(__err)	((__err) & DSISR_BAD_FAULT_32S)
  #endif
  
+#ifdef CONFIG_PER_VMA_LOCK
+bool arch_vma_access_error(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+	int is_exec = TRAP(vmf->regs) == INTERRUPT_INST_STORAGE;
+	int is_write = page_fault_is_write(vmf->fault_code);
+
+	if (unlikely(access_pkey_error(is_write, is_exec,
+				(vmf->fault_code & DSISR_KEYFAULT), vma)))
+		return true;
+
+	if (unlikely(access_error(is_write, is_exec, vma)))
+		return true;
+	return false;
+}
+#endif
+
  /*
   * For 600- and 800-family processors, the error_code parameter is DSISR
   * for a data fault, SRR1 for an instruction fault.
@@ -407,12 +423,18 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
  {
  	struct vm_area_struct * vma;
  	struct mm_struct *mm = current->mm;
-	unsigned int flags = FAULT_FLAG_DEFAULT;
  	int is_exec = TRAP(regs) == INTERRUPT_INST_STORAGE;
  	int is_user = user_mode(regs);
  	int is_write = page_fault_is_write(error_code);
  	vm_fault_t fault, major = 0;
  	bool kprobe_fault = kprobe_page_fault(regs, 11);
+	struct vm_fault vmf = {
+		.real_address = address,
+		.fault_code = error_code,
+		.regs = regs,
+		.flags = FAULT_FLAG_DEFAULT,
+	};
+
  
  	if (unlikely(debugger_fault_handler(regs) || kprobe_fault))
  		return 0;
@@ -463,45 +485,21 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
  	 * mmap_lock held
  	 */
  	if (is_user)
-		flags |= FAULT_FLAG_USER;
+		vmf.flags |= FAULT_FLAG_USER;
  	if (is_write)
-		flags |= FAULT_FLAG_WRITE;
+		vmf.flags |= FAULT_FLAG_WRITE;
  	if (is_exec)
-		flags |= FAULT_FLAG_INSTRUCTION;
+		vmf.flags |= FAULT_FLAG_INSTRUCTION;
  
-	if (!(flags & FAULT_FLAG_USER))
-		goto lock_mmap;
-
-	vma = lock_vma_under_rcu(mm, address);
-	if (!vma)
-		goto lock_mmap;
-
-	if (unlikely(access_pkey_error(is_write, is_exec,
-				       (error_code & DSISR_KEYFAULT), vma))) {
-		vma_end_read(vma);
-		goto lock_mmap;
-	}
-
-	if (unlikely(access_error(is_write, is_exec, vma))) {
-		vma_end_read(vma);
-		goto lock_mmap;
-	}
-
-	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
-	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
-		vma_end_read(vma);
-
-	if (!(fault & VM_FAULT_RETRY)) {
-		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
+	fault = try_vma_locked_page_fault(&vmf);
+	if (fault == VM_FAULT_NONE)
+		goto retry;
+	if (!(fault & VM_FAULT_RETRY))
  		goto done;
-	}
-	count_vm_vma_lock_event(VMA_LOCK_RETRY);
  
  	if (fault_signal_pending(fault, regs))
  		return user_mode(regs) ? 0 : SIGBUS;
  
-lock_mmap:
-
  	/* When running in the kernel we expect faults to occur only to
  	 * addresses in user space.  All other faults represent errors in the
  	 * kernel and should generate an OOPS.  Unfortunately, in the case of an
@@ -528,7 +526,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
  	 * make sure we exit gracefully rather than endlessly redo
  	 * the fault.
  	 */
-	fault = handle_mm_fault(vma, address, flags, regs);
+	fault = handle_mm_fault(vma, address, vmf.flags, regs);
  
  	major |= fault & VM_FAULT_MAJOR;
  
@@ -544,7 +542,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
  	 * case.
  	 */
  	if (unlikely(fault & VM_FAULT_RETRY)) {
-		flags |= FAULT_FLAG_TRIED;
+		vmf.flags |= FAULT_FLAG_TRIED;
  		goto retry;
  	}
  
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