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Subject[PATCH v2 2/2] arm64/mm: add speculative page fault
Date
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>
---
v2:
move find_vma() to do_page_fault()
remove IS_ENABLED()
remove fault != VM_FAULT_SIGSEGV check
initilize vma = NULL
---
arch/arm64/mm/fault.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 4165485..efd5956 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -320,14 +320,12 @@ 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 int __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 task_struct *tsk)
{
- struct vm_area_struct *vma;
int fault;

- vma = find_vma(mm, addr);
fault = VM_FAULT_BADMAP;
if (unlikely(!vma))
goto out;
@@ -371,6 +369,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
int fault, major = 0;
unsigned long vm_flags = VM_READ | VM_WRITE;
unsigned int mm_flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+ struct vm_area_struct *vma = NULL;

if (notify_page_fault(regs, esr))
return 0;
@@ -410,6 +409,16 @@ 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) {
+ perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, addr);
+ 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,
* we can bug out early if this is from code which shouldn't.
@@ -431,7 +440,10 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
#endif
}

- fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk);
+ if (!vma || !can_reuse_spf_vma(vma, addr))
+ vma = find_vma(mm, addr);
+
+ fault = __do_page_fault(vma, addr, mm_flags, vm_flags, tsk);
major |= fault & VM_FAULT_MAJOR;

if (fault & VM_FAULT_RETRY) {
@@ -454,11 +466,20 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
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;
}
}
up_read(&mm->mmap_sem);

+done:
+
/*
* Handle the "normal" (no error) case first.
*/
--
1.9.1
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