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Thu, 02 Feb 2023 03:20:58 -0800 (PST) Received: from redhat.com ([46.136.252.173]) by smtp.gmail.com with ESMTPSA id q10-20020a05600c330a00b003cffd3c3d6csm4348915wmp.12.2023.02.02.03.20.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Feb 2023 03:20:58 -0800 (PST) From: Juan Quintela To: David Hildenbrand Cc: qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Peter Xu , Andrey Gruzdev Subject: Re: [PATCH v1 5/5] migration/ram: Optimize ram_write_tracking_start() for RamDiscardManager In-Reply-To: <20230105124528.93813-6-david@redhat.com> (David Hildenbrand's message of "Thu, 5 Jan 2023 13:45:28 +0100") References: <20230105124528.93813-1-david@redhat.com> <20230105124528.93813-6-david@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) Date: Thu, 02 Feb 2023 12:20:57 +0100 Message-ID: <87fsboqpnq.fsf@secure.mitica> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=quintela@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, 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_H2=-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: , Reply-To: quintela@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org David Hildenbrand wrote: > ram_block_populate_read() already optimizes for RamDiscardManager. > However, ram_write_tracking_start() will still try protecting discarded > memory ranges. > > Let's optimize, because discarded ranges don't map any pages and > > (1) For anonymous memory, trying to protect using uffd-wp without a mapped > page is ignored by the kernel and consequently a NOP. > > (2) For shared/file-backed memory, we will fill present page tables in the > range with PTE markers. However, we will even allocate page tables > just to fill them with unnecessary PTE markers and effectively > waste memory. > > So let's exclude these ranges, just like ram_block_populate_read() > already does. > > Signed-off-by: David Hildenbrand Reviewed-by: Juan Quintela