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[2003:cb:c704:7c00:aaa9:2ce5:5aa0:f736]) by smtp.gmail.com with ESMTPSA id h9-20020a7bc929000000b003974a3af623sm9688836wml.17.2022.05.30.00.41.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 May 2022 00:41:23 -0700 (PDT) Message-ID: <0d266c61-605d-ce0c-4274-b0c7e10f845a@redhat.com> Date: Mon, 30 May 2022 09:41:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Content-Language: en-US To: zhenwei pi , Peter Xu , Jue Wang Cc: Andrew Morton , jasowang@redhat.com, LKML , Linux MM , mst@redhat.com, =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= , Paolo Bonzini , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org References: <24a95dea-9ea6-a904-7c0b-197961afa1d1@bytedance.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH 0/3] recover hardware corrupted page by virtio balloon In-Reply-To: <24a95dea-9ea6-a904-7c0b-197961afa1d1@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 27.05.22 08:32, zhenwei pi wrote: > On 5/27/22 02:37, Peter Xu wrote: >> On Wed, May 25, 2022 at 01:16:34PM -0700, Jue Wang wrote: >>> The hypervisor _must_ emulate poisons identified in guest physical >>> address space (could be transported from the source VM), this is to >>> prevent silent data corruption in the guest. With a paravirtual >>> approach like this patch series, the hypervisor can clear some of the >>> poisoned HVAs knowing for certain that the guest OS has isolated the >>> poisoned page. I wonder how much value it provides to the guest if the >>> guest and workload are _not_ in a pressing need for the extra KB/MB >>> worth of memory. >> >> I'm curious the same on how unpoisoning could help here. The reasoning >> behind would be great material to be mentioned in the next cover letter. >> >> Shouldn't we consider migrating serious workloads off the host already >> where there's a sign of more severe hardware issues, instead? >> >> Thanks, >> > > I'm maintaining 1000,000+ virtual machines, from my experience: > UE is quite unusual and occurs randomly, and I did not hit UE storm case > in the past years. The memory also has no obvious performance drop after > hitting UE. > > I hit several CE storm case, the performance memory drops a lot. But I > can't find obvious relationship between UE and CE. > > So from the point of my view, to fix the corrupted page for VM seems > good enough. And yes, unpoisoning several pages does not help > significantly, but it is still a chance to make the virtualization better. > I'm curious why we should care about resurrecting a handful of poisoned pages in a VM. The cover letter doesn't touch on that. IOW, I'm missing the motivation why we should add additional code+complexity to unpoison pages at all. If we're talking about individual 4k pages, it's certainly sub-optimal, but does it matter in practice? I could understand if we're losing megabytes of memory. But then, I assume the workload might be seriously harmed either way already? I assume when talking about "the performance memory drops a lot", you imply that this patch set can mitigate that performance drop? But why do you see a performance drop? Because we might lose some possible THP candidates (in the host or the guest) and you want to plug does holes? I assume you'll see a performance drop simply because poisoning memory is expensive, including migrating pages around on CE. If you have some numbers to share, especially before/after this change, that would be great. -- Thanks, David / dhildenb