Hi Ganesh,
I was looking at evaluating speculative page fault handling on arm64 and
noticed your patch.
Some comments below -
Ganesh Mahendran <opensource.ganesh@xxxxxxxxx> writes:
> 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@xxxxxxxxx>
> ---
> This patch is on top of Laurent's v10 spf
> ---
> arch/arm64/mm/fault.c | 38 +++++++++++++++++++++++++++++++++++---
> 1 file changed, 35 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 4165485..e7992a3 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -322,11 +322,13 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
>
> static int __do_page_fault(struct mm_struct *mm, unsigned long addr,
> unsigned int mm_flags, unsigned long vm_flags,
> - struct task_struct *tsk)
> + struct task_struct *tsk, struct vm_area_struct *vma)
> {
> - struct vm_area_struct *vma;
> int fault;
>
> + if (!vma || !can_reuse_spf_vma(vma, addr))
> + vma = find_vma(mm, addr);
> +
It would be better to move this hunk to do_page_fault().
It'll help localise the fact that handle_speculative_fault() is a
stateful call which needs a corresponding can_reuse_spf_vma() to
properly update the vma reference counting.
> vma = find_vma(mm, addr);
Remember to drop this call in the next version. As it stands the call
the find_vma() needlessly gets duplicated.
> fault = VM_FAULT_BADMAP;
> if (unlikely(!vma))
> @@ -371,6 +373,7 @@ 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;
>
> if (notify_page_fault(regs, esr))
> return 0;
> @@ -409,6 +412,25 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>
> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
>
> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT)) {
You don't need the IS_ENABLED() check. The alternate implementation of
handle_speculative_fault() when CONFIG_SPECULATIVE_PAGE_FAULT is not
enabled takes care of this.
> + fault = handle_speculative_fault(mm, addr, mm_flags, &vma);
> + /*
> + * Page fault is done if VM_FAULT_RETRY is not returned.
> + * But if the memory protection keys are active, we don't know
> + * if the fault is due to key mistmatch or due to a
> + * classic protection check.
> + * To differentiate that, we will need the VMA we no
> + * more have, so let's retry with the mmap_sem held.
> + */
As there is no support for memory protection keys on arm64 most of this
comment can be dropped.
> + if (fault != VM_FAULT_RETRY &&
> + fault != VM_FAULT_SIGSEGV) {
Not sure if you need the VM_FAULT_SIGSEGV here.
> + perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, addr);
> + goto done;
> + }
> + } else {
> + vma = NULL;
> + }
> +
If vma is initiliased to NULL during declaration, the else part can be
dropped.
> /*
> * As per x86, we may deadlock here. However, since the kernel only
> * validly references user space from well defined areas of the code,
> @@ -431,7 +453,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
> #endif
> }
>
> - fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk);
> + fault = __do_page_fault(mm, addr, mm_flags, vm_flags, tsk, vma);
> major |= fault & VM_FAULT_MAJOR;
>
> if (fault & VM_FAULT_RETRY) {
> @@ -454,11 +476,21 @@ 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.
> + */
> + if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
> + vma = NULL;
Please drop the IS_ENABLED() check.
Thanks,
Punit
> +
> goto retry;
> }
> }
> up_read(&mm->mmap_sem);
>
> +done:
> +
> /*
> * Handle the "normal" (no error) case first.
> */
