linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Peter Xu <peterx@redhat.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>,
	linux-kernel@vger.kernel.org,
	Gerald Schaefer <gerald.schaefer@de.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH 19/25] mm/s390: Use mm_fault_accounting()
Date: Wed, 17 Jun 2020 12:44:13 -0400	[thread overview]
Message-ID: <20200617164413.GG76766@xz-x1> (raw)
In-Reply-To: <8bd8dcf6-f2f0-d44e-9bf8-6fd4fe299aa9@de.ibm.com>

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


  reply	other threads:[~2020-06-17 16:44 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-15 22:15 [PATCH 00/25] mm: Page fault accounting cleanups Peter Xu
2020-06-15 22:15 ` [PATCH 01/25] mm/um: Fix extra accounting for page fault retries Peter Xu
2020-06-15 22:15 ` [PATCH 02/25] mm: Introduce mm_fault_accounting() Peter Xu
2020-06-15 22:32   ` Linus Torvalds
2020-06-15 23:19     ` Peter Xu
2020-06-16 19:00       ` Andrew Morton
2020-06-17 16:26         ` Peter Xu
2020-06-15 22:15 ` [PATCH 03/25] mm/alpha: Use mm_fault_accounting() Peter Xu
2020-06-15 22:15 ` [PATCH 04/25] mm/arc: " Peter Xu
2020-06-15 22:15 ` [PATCH 05/25] mm/arm: " Peter Xu
2020-06-15 22:15 ` [PATCH 06/25] mm/arm64: " Peter Xu
2020-06-16  7:43   ` Will Deacon
2020-06-16 15:59     ` Peter Xu
2020-06-15 22:15 ` [PATCH 07/25] mm/csky: " Peter Xu
2020-06-17  7:04   ` Guo Ren
2020-06-17 15:49     ` Peter Xu
2020-06-17 17:53       ` Linus Torvalds
2020-06-17 19:58         ` Peter Xu
2020-06-17 20:15           ` Linus Torvalds
2020-06-18 14:38             ` Peter Xu
2020-06-18 17:15               ` Linus Torvalds
2020-06-18 21:24                 ` Peter Xu
2020-06-18 22:28                   ` Peter Xu
2020-06-18 22:59                     ` Linus Torvalds
2020-06-15 22:15 ` [PATCH 08/25] mm/hexagon: " Peter Xu
2020-06-15 22:15 ` [PATCH 09/25] mm/ia64: " Peter Xu
2020-06-15 22:15 ` [PATCH 10/25] mm/m68k: " Peter Xu
2020-06-15 22:15 ` [PATCH 11/25] mm/microblaze: " Peter Xu
2020-06-15 22:15 ` [PATCH 12/25] mm/mips: " Peter Xu
2020-06-15 22:15 ` [PATCH 13/25] mm/nds32: " Peter Xu
2020-06-17  1:05   ` Greentime Hu
2020-06-15 22:15 ` [PATCH 14/25] mm/nios2: " Peter Xu
2020-06-15 22:15 ` [PATCH 15/25] mm/openrisc: " Peter Xu
2020-06-16 18:11   ` Stafford Horne
2020-06-15 22:15 ` [PATCH 16/25] mm/parisc: " Peter Xu
2020-06-15 22:15 ` [PATCH 17/25] mm/powerpc: " Peter Xu
2020-06-15 22:16 ` [PATCH 18/25] mm/riscv: " Peter Xu
2020-06-18 23:49   ` Palmer Dabbelt
2020-06-19  0:12     ` Peter Xu
2020-06-15 22:23 ` [PATCH 19/25] mm/s390: " Peter Xu
2020-06-16 15:59   ` Alexander Gordeev
2020-06-16 16:35     ` Peter Xu
2020-06-17  6:19       ` Christian Borntraeger
2020-06-17 16:06         ` Peter Xu
2020-06-17 16:14           ` Christian Borntraeger
2020-06-17 16:44             ` Peter Xu [this message]
2020-06-15 22:23 ` [PATCH 20/25] mm/sh: " Peter Xu
2020-07-20 21:25   ` Rich Felker
2020-07-20 22:05     ` Peter Xu
2020-06-15 22:23 ` [PATCH 21/25] mm/sparc32: " Peter Xu
2020-06-15 22:23 ` [PATCH 22/25] mm/sparc64: " Peter Xu
2020-06-15 22:23 ` [PATCH 23/25] mm/unicore32: " Peter Xu
2020-06-15 22:23 ` [PATCH 24/25] mm/x86: " Peter Xu
2020-06-15 22:23 ` [PATCH 25/25] mm/xtensa: " Peter Xu
2020-06-15 23:13   ` Max Filippov
2020-06-16 18:55 ` [PATCH 00/25] mm: Page fault accounting cleanups Linus Torvalds
2020-06-16 21:03   ` Peter Xu
2020-06-17  0:55   ` Michael Ellerman
2020-06-17  8:04     ` Will Deacon
2020-06-17 16:10       ` Peter Xu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200617164413.GG76766@xz-x1 \
    --to=peterx@redhat.com \
    --cc=aarcange@redhat.com \
    --cc=agordeev@linux.ibm.com \
    --cc=akpm@linux-foundation.org \
    --cc=borntraeger@de.ibm.com \
    --cc=gerald.schaefer@de.ibm.com \
    --cc=gor@linux.ibm.com \
    --cc=heiko.carstens@de.ibm.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-s390@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    --subject='Re: [PATCH 19/25] mm/s390: Use mm_fault_accounting()' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
on how to clone and mirror all data and code used for this inbox