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[99.241.198.116]) by smtp.gmail.com with ESMTPSA id u10-20020a02b1ca000000b00339da678a7csm158539jah.78.2022.06.23.13.18.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Jun 2022 13:18:11 -0700 (PDT) Date: Thu, 23 Jun 2022 16:18:09 -0400 From: Peter Xu To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paolo Bonzini , Andrew Morton , David Hildenbrand , "Dr . David Alan Gilbert" , Andrea Arcangeli , Linux MM Mailing List Subject: Re: [PATCH 4/4] kvm/x86: Allow to respond to generic signals during slow page faults Message-ID: References: <20220622213656.81546-1-peterx@redhat.com> <20220622213656.81546-5-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 23, 2022 at 08:07:00PM +0000, Sean Christopherson wrote: > On Thu, Jun 23, 2022, Peter Xu wrote: > > Hi, Sean, > > > > On Thu, Jun 23, 2022 at 02:46:08PM +0000, Sean Christopherson wrote: > > > On Wed, Jun 22, 2022, Peter Xu wrote: > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > > > index e92f1ab63d6a..b39acb7cb16d 100644 > > > > --- a/arch/x86/kvm/mmu/mmu.c > > > > +++ b/arch/x86/kvm/mmu/mmu.c > > > > @@ -3012,6 +3012,13 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn) > > > > static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, > > > > unsigned int access) > > > > { > > > > + /* NOTE: not all error pfn is fatal; handle intr before the other ones */ > > > > + if (unlikely(is_intr_pfn(fault->pfn))) { > > > > + vcpu->run->exit_reason = KVM_EXIT_INTR; > > > > + ++vcpu->stat.signal_exits; > > > > + return -EINTR; > > > > + } > > > > + > > > > /* The pfn is invalid, report the error! */ > > > > if (unlikely(is_error_pfn(fault->pfn))) > > > > return kvm_handle_bad_page(vcpu, fault->gfn, fault->pfn); > > > > @@ -4017,6 +4024,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) > > > > } > > > > } > > > > > > > > + /* Allow to respond to generic signals in slow page faults */ > > > > > > "slow" is being overloaded here. The previous call __gfn_to_pfn_memslot() will > > > end up in hva_to_pfn_slow(), but because of passing a non-null async it won't wait. > > > This code really should have a more extensive comment irrespective of the interruptible > > > stuff, now would be a good time to add that. > > > > Yes I agree, especially the "async" parameter along with "atomic" makes it > > even more confusing as you said. But isn't that also means the "slow" here > > is spot-on? I mean imho it's the "elsewhere" needs cleanup not this > > comment itself since it's really stating the fact that this is the real > > slow path? > > No, because atomic=false here, i.e. KVM will try hva_to_pfn_slow() if hva_to_pfn_fast() > fails. So even if we agree that the "wait for IO" path is the true slow path, > when reading KVM code the vast, vast majority of developers will associate "slow" > with hva_to_pfn_slow(). Okay. I think how we define slow matters, here my take is "when a major fault happens" (as defined in the mm term), but probably that definition is a bit far away from kvm as the hypervisor level indeed. > > > Or any other suggestions greatly welcomed on how I should improve this > > comment. > > Something along these lines? > > /* > * Allow gup to bail on pending non-fatal signals when it's also allowed > * to wait for IO. Note, gup always bails if it is unable to quickly > * get a page and a fatal signal, i.e. SIGKILL, is pending. > */ Taken. > > > > > > > > Comments aside, isn't this series incomplete from the perspective that there are > > > still many flows where KVM will hang if gfn_to_pfn() gets stuck in gup? E.g. if > > > KVM is retrieving a page pointed at by vmcs12. > > > > Right. The thing is I'm not confident I can make it complete easily in one > > shot.. > > > > I mentioned some of that in cover letter or commit message of patch 1, in > > that I don't think all the gup call sites are okay with being interrupted > > by a non-fatal signal. > > > > So what I want to do is doing it step by step, at least by introducing > > FOLL_INTERRUPTIBLE and having one valid user of it that covers a very valid > > use case. I'm also pretty sure the page fault path is really the most > > cases that will happen with GUP, so it already helps in many ways for me > > when running with a patched kernel. > > > > So when the complete picture is non-trivial to achieve in one shot, I think > > this could be one option we go for. With the facility (and example code on > > x86 slow page fault) ready, hopefully we could start to convert many other > > call sites to be signal-aware, outside page faults, or even outside x86, > > because it's really a more generic problem, which I fully agree. > > > > Does it sound reasonable to you? > > Yep. In fact, I'd be totally ok keeping this to just the page fault path. I > missed one cruicial detail on my first read through: gup already bails on SIGKILL, > it's only these technically-not-fatal signals that gup ignores by default. In > other words, using FOLL_INTERRUPTIBLE is purely opportunsitically as userspace > can always resort to SIGKILL if the VM really needs to die. > > It would be very helpful to explicit call that out in the changelog, that way > it's (hopefully) clear that KVM uses FOLL_INTERRUPTIBLE to be user friendly when > it's easy to do so, and that it's not required for correctness/robustness. Yes that's the case, sigkill is special. I can mention that somewhere in the cover letter too besides the comment you suggested above. Thanks, -- Peter Xu