On Wed, Jun 17, 2020 at 06:14:52PM +0200, Christian Borntraeger wrote:
>
>
> On 17.06.20 18:06, Peter Xu wrote:
> > Hi, Christian,
> >
> > On Wed, Jun 17, 2020 at 08:19:29AM +0200, Christian Borntraeger wrote:
> >>
> >>
> >> On 16.06.20 18:35, Peter Xu wrote:
> >>> Hi, Alexander,
> >>>
> >>> On Tue, Jun 16, 2020 at 05:59:33PM +0200, Alexander Gordeev wrote:
> >>>>> @@ -489,21 +489,7 @@ static inline vm_fault_t do_exception(struct
> >>>>> pt_regs *regs, int access)
> >>>>> if (unlikely(fault & VM_FAULT_ERROR))
> >>>>> goto out_up;
> >>>>>
> >>>>> - /*
> >>>>> - * Major/minor page fault accounting is only done on the
> >>>>> - * initial attempt. If we go through a retry, it is extremely
> >>>>> - * likely that the page will be found in page cache at that
> >>>>> point.
> >>>>> - */
> >>>>> if (flags & FAULT_FLAG_ALLOW_RETRY) {
> >>>>> - if (fault & VM_FAULT_MAJOR) {
> >>>>> - tsk->maj_flt++;
> >>>>> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
> >>>>> - regs, address);
> >>>>> - } else {
> >>>>> - tsk->min_flt++;
> >>>>> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
> >>>>> - regs, address);
> >>>>> - }
> >>>>> if (fault & VM_FAULT_RETRY) {
> >>>>> if (IS_ENABLED(CONFIG_PGSTE) && gmap &&
> >>>>> (flags & FAULT_FLAG_RETRY_NOWAIT)) {
> >
> > [1]
> >
> >>>>
> >>>> Seems like the call to mm_fault_accounting() will be missed if
> >>>> we entered here with FAULT_FLAG_RETRY_NOWAIT flag set, since it
> >>>> jumps to "out_up"...
> >>>
> >>> This is true as a functional change. However that also means that we've
> >>> got a
> >>> VM_FAULT_RETRY, which hints that this fault has been requested to retry
> >>> rather
> >>> than handled correctly (for instance, due to some try_lock failed during
> >>> the
> >>> fault process).
> >>>
> >>> To me, that case should not be counted as a page fault at all? Or we
> >>> might get
> >>> the same duplicated accounting when the page fault retried from a higher
> >>> stack.
> >>>
> >>> Thanks
> >>
> >> This case below (the one with the gmap) is the KVM case for doing a so
> >> called
> >> pseudo page fault to our guests. (we notify our guests about major host
> >> page
> >> faults and let it reschedule to something else instead of halting the
> >> vcpu).
> >> This is being resolved with either gup or fixup_user_fault asynchronously
> >> by
> >> KVM code (this can also be sync when the guest does not match some
> >> conditions)
> >> We do not change the counters in that code as far as I can tell so we
> >> should
> >> continue to do it here.
> >>
> >> (see arch/s390/kvm/kvm-s390.c
> >> static int vcpu_post_run(struct kvm_vcpu *vcpu, int exit_reason)
> >> {
> >> [...]
> >> } else if (current->thread.gmap_pfault) {
> >> trace_kvm_s390_major_guest_pfault(vcpu);
> >> current->thread.gmap_pfault = 0;
> >> if (kvm_arch_setup_async_pf(vcpu))
> >> return 0;
> >> return kvm_arch_fault_in_page(vcpu,
> >> current->thread.gmap_addr, 1);
> >> }
> >
> > Please correct me if I'm wrong... but I still think what this patch does is
> > the
> > right thing to do.
> >
> > Note again that IMHO when reached [1] above it means the page fault is not
> > handled correctly so we need to fallback to KVM async page fault, then we
> > shouldn't increment the accountings until it's finally handled correctly.
> > That
> > final accounting should be done in the async pf path in gup code where the
> > page
> > fault is handled:
> >
> > kvm_arch_fault_in_page
> > gmap_fault
> > fixup_user_fault
> >
> > Where in fixup_user_fault() we have:
> >
> > if (tsk) {
> > if (major)
> > tsk->maj_flt++;
> > else
> > tsk->min_flt++;
> > }
> >
>
> Right that case does work. Its the case where we do not inject a pseudo
> pagefault and
> instead fall back to synchronous fault-in.
> What is about the other case:
>
> kvm_setup_async_pf
> ->workqueue
> async_pf_execute
> get_user_pages_remote
>
> Does get_user_pages_remote do the accounting as well? I cant see that.
>
It should, with:
get_user_pages_remote
__get_user_pages_remote
__get_user_pages_locked
__get_user_pages
faultin_page
Where the accounting is done in faultin_page().
We probably need to also move the accounting in faultin_page() to be after the
retry check too, but that should be irrelevant to this patch.
Thanks!
--
Peter Xu
