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[91.12.99.11]) by smtp.gmail.com with ESMTPSA id m9sm5144546wrz.75.2021.08.05.01.10.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Aug 2021 01:10:39 -0700 (PDT) Subject: Re: [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination To: Peter Xu References: <20210730085249.8246-1-david@redhat.com> <20210730085249.8246-7-david@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: <14437dd7-88b2-94df-a70f-e5c38ca861e1@redhat.com> Date: Thu, 5 Aug 2021 10:10:38 +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=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@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, 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 05.08.21 02:04, Peter Xu wrote: > On Fri, Jul 30, 2021 at 10:52:48AM +0200, 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)); >> + >> + /* >> + * Discarded pages (via RamDiscardManager) are never migrated. On unlikely >> + * access, place a zeropage, which will also set the relevant bits in the >> + * recv_bitmap accordingly, so we won't try placing a zeropage twice. >> + * >> + * Checking a single bit is sufficient to handle pagesize > TPS as either >> + * all relevant bits are set or not. >> + */ >> + assert(QEMU_IS_ALIGNED(start, qemu_ram_pagesize(rb))); > > Is this check for ramblock_page_is_discarded()? If so, shall we move this into > it, e.g. after memory_region_has_ram_discard_manager() returned true? > It has to hold true also when calling migrate_send_rp_req_pages(). Both callers -- postcopy_request_shared_page() and postcopy_ram_fault_thread() properly align the offset down (but not the host address). This check is just to make sure we don't mess up in the future. -- Thanks, David / dhildenb