FROMLIST: arm64/mm: add speculative page fault

This patch enables the speculative page fault on the arm64
architecture.

I completed spf porting in 4.9. From the test result,
we can see app launching time improved by about 10% in average.
For the apps which have more than 50 threads, 15% or even more
improvement can be got.

Signed-off-by: Ganesh Mahendran <opensource.ganesh@gmail.com>
Change-Id: Ib7c8b2e354800b5023e6c6400448a6d40aaf89c8
Link: https://patchwork.kernel.org/project/linux-mm/patch/1525247672-2165-2-git-send-email-opensource.ganesh@gmail.com/
Bug: 161210518
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>

arch/arm64/mm/fault.c

@@ -464,11 +464,10 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
 #define VM_FAULT_BADMAP		0x010000
 #define VM_FAULT_BADACCESS	0x020000
 
-static vm_fault_t __do_page_fault(struct mm_struct *mm, unsigned long addr,
+static int __do_page_fault(struct vm_area_struct *vma, unsigned long addr,
 				  unsigned int mm_flags, unsigned long vm_flags,
 				  struct pt_regs *regs)
 {
-	struct vm_area_struct *vma = find_vma(mm, addr);
 
 	if (unlikely(!vma))
 		return VM_FAULT_BADMAP;
@@ -515,6 +514,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	vm_fault_t fault;
 	unsigned long vm_flags = VM_ACCESS_FLAGS;
 	unsigned int mm_flags = FAULT_FLAG_DEFAULT;
+	struct vm_area_struct *vma = NULL;
 
 	if (kprobe_page_fault(regs, esr))
 		return 0;
@@ -554,6 +554,14 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
 
+	/*
+	 * let's try a speculative page fault without grabbing the
+	 * mmap_sem.
+	 */
+	fault = handle_speculative_fault(mm, addr, mm_flags, &vma);
+	if (fault != VM_FAULT_RETRY)
+		goto done;
+
 	/*
 	 * As per x86, we may deadlock here. However, since the kernel only
 	 * validly references user space from well defined areas of the code,
@@ -578,7 +586,9 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 #endif
 	}
 
-	fault = __do_page_fault(mm, addr, mm_flags, vm_flags, regs);
+	if (!vma || !can_reuse_spf_vma(vma, addr))
+		vma = find_vma(mm, addr);
+	fault = __do_page_fault(vma, addr, mm_flags, vm_flags, regs);
 
 	/* Quick path to respond to signals */
 	if (fault_signal_pending(fault, regs)) {
@@ -590,11 +600,20 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	if (fault & VM_FAULT_RETRY) {
 		if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
 			mm_flags |= FAULT_FLAG_TRIED;
+
+			/*
+			 * Do not try to reuse this vma and fetch it
+			 * again since we will release the mmap_sem.
+			 */
+			vma = NULL;
+
 			goto retry;
 		}
 	}
 	mmap_read_unlock(mm);
 
+done:
+
 	/*
 	 * Handle the "normal" (no error) case first.
 	 */