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[95.127.154.109]) by smtp.gmail.com with ESMTPSA id 140sm4977580wmb.43.2021.08.05.01.17.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Aug 2021 01:17:34 -0700 (PDT) Subject: Re: [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination To: David Hildenbrand , qemu-devel@nongnu.org References: <20210730085249.8246-1-david@redhat.com> <20210730085249.8246-7-david@redhat.com> <5f8c6173-046d-9fc2-c649-93ede45ca77d@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <2c8d80ad-f171-7d5f-3235-92f02fa174b3@redhat.com> Date: Thu, 5 Aug 2021 10:17:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=philmd@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=philmd@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.132, 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: Alex Williamson , Eduardo Habkost , Juan Quintela , "Michael S. Tsirkin" , "Dr. David Alan Gilbert" , Peter Xu , Andrey Gruzdev , Pankaj Gupta , teawater , Paolo Bonzini , Marek Kedzierski , Wei Yang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 8/5/21 10:07 AM, David Hildenbrand wrote: > On 05.08.21 09:48, Philippe Mathieu-Daudé wrote: >> On 7/30/21 10:52 AM, David Hildenbrand wrote: >>> Currently, when someone (i.e., the VM) accesses discarded parts inside a >>> RAMBlock with a RamDiscardManager managing the corresponding mapped >>> memory >>> region, postcopy will request migration of the corresponding page >>> from the >>> source. The source, however, will never answer, because it refuses to >>> migrate such pages with undefined content ("logically unplugged"): the >>> pages are never dirty, and get_queued_page() will consequently skip >>> processing these postcopy requests. >>> >>> Especially reading discarded ("logically unplugged") ranges is >>> supposed to >>> work in some setups (for example with current virtio-mem), although it >>> barely ever happens: still, not placing a page would currently stall the >>> VM, as it cannot make forward progress. >>> >>> Let's check the state via the RamDiscardManager (the state e.g., >>> of virtio-mem is migrated during precopy) and avoid sending a request >>> that will never get answered. Place a fresh zero page instead to keep >>> the VM working. This is the same behavior that would happen >>> automatically without userfaultfd being active, when accessing virtual >>> memory regions without populated pages -- "populate on demand". >>> >>> For now, there are valid cases (as documented in the virtio-mem spec) >>> where >>> a VM might read discarded memory; in the future, we will disallow that. >>> Then, we might want to handle that case differently, e.g., warning the >>> user that the VM seems to be mis-behaving. >>> >>> Signed-off-by: David Hildenbrand >>> --- >>>   migration/postcopy-ram.c | 31 +++++++++++++++++++++++++++---- >>>   migration/ram.c          | 21 +++++++++++++++++++++ >>>   migration/ram.h          |  1 + >>>   3 files changed, 49 insertions(+), 4 deletions(-) >>> >>> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c >>> index 2e9697bdd2..38cdfc09c3 100644 >>> --- a/migration/postcopy-ram.c >>> +++ b/migration/postcopy-ram.c >>> @@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, >>>       return ret; >>>   } >>>   +static int postcopy_request_page(MigrationIncomingState *mis, >>> RAMBlock *rb, >>> +                                 ram_addr_t start, uint64_t haddr) >>> +{ >>> +    void *aligned = (void *)(uintptr_t)(haddr & >>> -qemu_ram_pagesize(rb)); >> >>    void *aligned = QEMU_ALIGN_PTR_DOWN(haddr, qemu_ram_pagesize(rb))); >> > > Does not compile as haddr is not a pointer. I suppose the typeof() fails? /* n-byte align pointer down */ #define QEMU_ALIGN_PTR_DOWN(p, n) \ ((typeof(p))QEMU_ALIGN_DOWN((uintptr_t)(p), (n))) > void *aligned = QEMU_ALIGN_PTR_DOWN((void *)haddr, qemu_ram_pagesize(rb))); > > should work. What about void *aligned = QEMU_ALIGN_DOWN(haddr, qemu_ram_pagesize(rb))); else void *aligned = (void *)QEMU_ALIGN_DOWN(haddr, qemu_ram_pagesize(rb))); I don't mind much the style you prefer, as long as it compiles :p But these QEMU_ALIGN_DOWN() macros make the review easier than (a & -b). > I can also add a patch to adjust a similar call in > migrate_send_rp_req_pages()! > > Thanks! >