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[95.127.154.109]) by smtp.gmail.com with ESMTPSA id t1sm5082222wrm.42.2021.08.05.01.21.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Aug 2021 01:21:26 -0700 (PDT) Subject: Re: [PATCH v3 7/7] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots To: David Hildenbrand , qemu-devel@nongnu.org References: <20210730085249.8246-1-david@redhat.com> <20210730085249.8246-8-david@redhat.com> <265427ef-ea74-e352-8148-7e4353af6ceb@redhat.com> From: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Message-ID: <025af202-759f-3d8b-b40a-dba346a36696@redhat.com> Date: Thu, 5 Aug 2021 10:21:19 +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: <265427ef-ea74-e352-8148-7e4353af6ceb@redhat.com> 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=170.10.133.124; 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, 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 10:11 AM, David Hildenbrand wrote: > On 05.08.21 10:04, Philippe Mathieu-Daudé wrote: >> On 7/30/21 10:52 AM, David Hildenbrand wrote: >>> We already don't ever migrate memory that corresponds to discarded >>> ranges >>> as managed by a RamDiscardManager responsible for the mapped memory >>> region >>> of the RAMBlock. >>> >>> virtio-mem uses this mechanism to logically unplug parts of a RAMBlock. >>> Right now, we still populate zeropages for the whole usable part of the >>> RAMBlock, which is undesired because: >>> >>> 1. Even populating the shared zeropage will result in memory getting >>>     consumed for page tables. >>> 2. Memory backends without a shared zeropage (like hugetlbfs and shmem) >>>     will populate an actual, fresh page, resulting in an unintended >>>     memory consumption. >>> >>> Discarded ("logically unplugged") parts have to remain discarded. As >>> these pages are never part of the migration stream, there is no need to >>> track modifications via userfaultfd WP reliably for these parts. >>> >>> Further, any writes to these ranges by the VM are invalid and the >>> behavior is undefined. >>> >>> Note that Linux only supports userfaultfd WP on private anonymous memory >>> for now, which usually results in the shared zeropage getting populated. >>> The issue will become more relevant once userfaultfd WP supports shmem >>> and hugetlb. >>> >>> Acked-by: Peter Xu >>> Signed-off-by: David Hildenbrand >>> --- >>>   migration/ram.c | 53 +++++++++++++++++++++++++++++++++++++++++-------- >>>   1 file changed, 45 insertions(+), 8 deletions(-) >>> >>> diff --git a/migration/ram.c b/migration/ram.c >>> index 01cea01774..fd5949734e 100644 >>> --- a/migration/ram.c >>> +++ b/migration/ram.c >>> @@ -1639,6 +1639,28 @@ out: >>>       return ret; >>>   } >>>   +static inline void populate_range(RAMBlock *block, hwaddr offset, >>> hwaddr size) >>> +{ >>> +    char *ptr = (char *) block->host; >>> + >>> +    for (; offset < size; offset += qemu_real_host_page_size) { >>> +        char tmp = *(ptr + offset); >>> + >>> +        /* Don't optimize the read out */ >>> +        asm volatile("" : "+r" (tmp)); >>> +    } >> >> This template is now used 3 times, a good opportunity to extract it as >> an (inline?) helper. >> > > Can you point me at the other users? Oops I got lost reviewing the series. > Isn't populate_range() the inline helper you are looking for? :) Indeed :) Being a bit picky, I'd appreciate if you split this patch in 2, extracting populate_range() as a first trivial step.