Messages in this thread | | | From | Punit Agrawal <> | Subject | Re: [PATCH 2/2] arm64/mm: add speculative page fault | Date | Wed, 02 May 2018 15:46:02 +0100 |
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Hi Ganesh,
I was looking at evaluating speculative page fault handling on arm64 and noticed your patch.
Some comments below -
Ganesh Mahendran <opensource.ganesh@gmail.com> 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@gmail.com> > --- > 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. > */
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