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[184.144.111.238]) by smtp.gmail.com with ESMTPSA id u11sm1952274qkk.72.2021.07.29.08.52.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Jul 2021 08:52:54 -0700 (PDT) Date: Thu, 29 Jul 2021 11:52:53 -0400 From: Peter Xu To: David Hildenbrand Subject: Re: [PATCH v2 5/6] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination Message-ID: References: <20210721092759.21368-1-david@redhat.com> <20210721092759.21368-6-david@redhat.com> <2ce949f2-6950-5404-70e4-73a2e41b4ca8@redhat.com> <124693ef-5595-85c9-da5a-8f8e6a827c19@redhat.com> MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.717, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , Juan Quintela , Pankaj Gupta , "Michael S. Tsirkin" , teawater , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Alex Williamson , Marek Kedzierski , Paolo Bonzini , Andrey Gruzdev , Wei Yang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jul 29, 2021 at 02:14:41PM +0200, David Hildenbrand wrote: > On 24.07.21 00:10, Peter Xu wrote: > > On Fri, Jul 23, 2021 at 09:01:42PM +0200, David Hildenbrand wrote: > > > It can happen in corner cases and is valid: with the current virtio-mem > > > spec, guests are allowed to read unplugged memory. This will, for example, > > > happen on older Linux guests when reading /proc/kcore or (with even older > > > guests) when dumping guest memory via kdump. These corner cases were the > > > main reason why the spec allows for it -- until we have guests properly > > > adjusted such that it won't happen even in corner cases. > > > > > > A future feature bit will disallow it for the guest: required for supporting > > > shmem/hugetlb cleanly. With that in place, I agree that we would want to > > > warn in this case! > > > > OK that makes sense; with the page_size change, feel free to add: > > I just realized that relying on the page_size would be wrong. > > We migrate TARGET_PAGE_SIZE chunks and the offset might not be page_size > aligned. So if we were to replace TARGET_PAGE_SIZE by rb->page_size, we > might accidentally cover a "too big" range. I'm wondering whether we should make the offset page size aligned instead. For example, note that postcopy_place_page_zero() should only take page_size aligned host addr or UFFDIO_COPY could fail (hugetlb doesn't support UFFDIO_ZEROPAGE yet). Btw, does virtio-mem supports hugetlbfs now? When with it, the smallest unit to plug/unplug would the huge page size (e.g., for 1g huge page, sounds not helpful to unplug 2M memory), am I right? -- Peter Xu