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[184.144.111.238]) by smtp.gmail.com with ESMTPSA id h68sm2223734qkf.126.2021.07.29.12.20.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Jul 2021 12:20:49 -0700 (PDT) Date: Thu, 29 Jul 2021 15:20:47 -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> <5ede7b00-1048-c124-e239-eeff21d9adb0@redhat.com> MIME-Version: 1.0 In-Reply-To: <5ede7b00-1048-c124-e239-eeff21d9adb0@redhat.com> 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=216.205.24.124; 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_H3=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 06:15:58PM +0200, David Hildenbrand wrote: > On 29.07.21 17:52, Peter Xu wrote: > > 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). > > That is true indeed. I'd assume in that case that we would get called with > the proper offset already, right? Because uffd would only report properly > aligned pages IIRC. Nop; it should return the faulted address. So postcopy_request_page() may need some alignment work, as it was handled in migrate_send_rp_req_pages(). > > > > > 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? > > It supports it to 99.9 % I'd say (especially with the dump/tpm/migration > fixes upstream). The units are handled properly: the unit is at least as big > as the page size. > > So with 1 GiB pages, you have a unit of 1 GiB. I see, thanks for confirming. Then fixing up the offset seems to be the right thing to do. Thanks, -- Peter Xu