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Wed, 23 Apr 2025 15:51:03 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFlKlclMBHJA/Aq1cMtVdWVreRKbQInuwAtGyBdUDmmTuGDVCa3g/KJFmT4JDyUa9M5V6HoCw== X-Received: by 2002:a05:620a:371d:b0:7c5:4eee:5409 with SMTP id af79cd13be357-7c956f4709amr83103085a.48.1745448663019; Wed, 23 Apr 2025 15:51:03 -0700 (PDT) Received: from x1.local ([85.131.185.92]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7c925ac5f78sm741133985a.65.2025.04.23.15.51.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 23 Apr 2025 15:51:01 -0700 (PDT) Date: Wed, 23 Apr 2025 18:50:58 -0400 From: Peter Xu To: Fabiano Rosas Cc: Prasad Pandit , qemu-devel@nongnu.org, berrange@redhat.com, Prasad Pandit Subject: Re: [PATCH v9 0/7] Allow to enable multifd and postcopy migration together Message-ID: References: <20250411114534.3370816-1-ppandit@redhat.com> <87ecxteym0.fsf@suse.de> <87bjswfeis.fsf@suse.de> <87y0vyepta.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87y0vyepta.fsf@suse.de> 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: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.294, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, Apr 17, 2025 at 01:05:37PM -0300, Fabiano Rosas wrote: > It's not that page faults happen during multifd. The page was already > sent during precopy, but multifd-recv didn't write to it, it just marked > the receivedmap. When postcopy starts, the page gets accessed and > faults. Since postcopy is on, the migration wants to request the page > from the source, but it's present in the receivedmap, so it doesn't > ask. No page ever comes and the code hangs waiting for the page fault to > be serviced (or potentially faults continuously? I'm not sure on the > details). I think your previous analysis is correct on the zero pages. I am not 100% sure if that's the issue but very likely. I tend to also agree with you that we could skip zero page optimization in multifd code when postcopy is enabled (maybe plus some comment right above..). Thanks, -- Peter Xu