Re: [RFC PATCH] mm, oom: distinguish blockable mode for mmu notifiers

From: Michal Hocko <mhocko@kernel.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	kvm@vger.kernel.org, "Radim Krčmář" <rkrcmar@redhat.com>,
	linux-mm@kvack.org, "Andrea Arcangeli" <aarcange@redhat.com>,
	"Jérôme Glisse" <jglisse@redhat.com>
Subject: Re: [RFC PATCH] mm, oom: distinguish blockable mode for mmu notifiers
Date: Mon, 25 Jun 2018 13:08:30 +0200	[thread overview]
Message-ID: <20180625110830.GJ28965@dhcp22.suse.cz> (raw)
In-Reply-To: <45e1a12c-280b-635a-fc76-716440f084ec@redhat.com>

On Mon 25-06-18 12:34:43, Paolo Bonzini wrote:
> On 25/06/2018 10:45, Michal Hocko wrote:
> > On Mon 25-06-18 10:10:18, Paolo Bonzini wrote:
> >> On 25/06/2018 09:57, Michal Hocko wrote:
> >>> On Sun 24-06-18 10:11:21, Paolo Bonzini wrote:
> >>>> On 22/06/2018 17:02, Michal Hocko wrote:
> >>>>> @@ -7215,6 +7216,8 @@ void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
> >>>>>  	apic_address = gfn_to_hva(kvm, APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
> >>>>>  	if (start <= apic_address && apic_address < end)
> >>>>>  		kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD);
> >>>>> +
> >>>>> +	return 0;
> >>>>
> >>>> This is wrong, gfn_to_hva can sleep.
> >>>
> >>> Hmm, I have tried to crawl the call chain and haven't found any
> >>> sleepable locks taken. Maybe I am just missing something.
> >>> __kvm_memslots has a complex locking assert. I do not see we would take
> >>> slots_lock anywhere from the notifier call path. IIUC that means that
> >>> users_count has to be zero at that time. I have no idea how that is
> >>> guaranteed.
> >>
> >> Nevermind, ENOCOFFEE.  This is gfn_to_hva, not gfn_to_pfn.  It only
> >> needs SRCU.
> > 
> > OK, so just the make sure I follow, the change above is correct?
> 
> Yes.  It's only gfn_to_pfn that calls get_user_pages and therefore can
> sleep.

OK, thanks for the confirmation!
-- 
Michal Hocko
SUSE Labs