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Wed, 12 Mar 2025 12:32:34 -0700 (PDT) X-Google-Smtp-Source: AGHT+IG21UoB4UhW6J7OnJXCWJmtkS+rArAGv2lRBmFk4GlTGcEtlkmQsqJSDwW+31DKN2CBG4Usgw== X-Received: by 2002:a05:620a:268e:b0:7c5:55f9:4bbf with SMTP id af79cd13be357-7c555f94e37mr2089054185a.7.1741807954664; Wed, 12 Mar 2025 12:32:34 -0700 (PDT) Received: from x1.local ([85.131.185.92]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7c54c8f99absm570901185a.117.2025.03.12.12.32.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Mar 2025 12:32:33 -0700 (PDT) Date: Wed, 12 Mar 2025 15:32:30 -0400 From: Peter Xu To: Nikita Kalyazin Cc: James Houghton , akpm@linux-foundation.org, pbonzini@redhat.com, shuah@kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, david@redhat.com, ryan.roberts@arm.com, quic_eberman@quicinc.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com Subject: Re: [RFC PATCH 0/5] KVM: guest_memfd: support for uffd missing Message-ID: References: <20250303133011.44095-1-kalyazin@amazon.com> <9e7536cc-211d-40ca-b458-66d3d8b94b4d@amazon.com> <7c304c72-1f9c-4a5a-910b-02d0f1514b01@amazon.com> MIME-Version: 1.0 In-Reply-To: <7c304c72-1f9c-4a5a-910b-02d0f1514b01@amazon.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 1be897vkGuHSaUlRD4s7YgLCREvsTqhAiIjgjJ7ANWk_1741807955 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 35D231C0004 X-Stat-Signature: de3s74sgf3nis33dxtgzjbwrbdx6r659 X-HE-Tag: 1741807957-771197 X-HE-Meta: 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 LPDmoTSn ATsAlll9FximwZJyLTbCftJLrxieaQDfCUkCK3DDXPmMVmItolYucrUU9IyxyEW0aIxuzZ7TBveumVSC5V5YIIWR65PPxl78VuyonHbfRZYWuFMckAk6m7cHrhMe2eV7sBpDoYE5jpxWlwnbkpuohR1S/3d5dU4kFlSqs0NTs48Mgb6jbQKjwlv6XJzZVvYFzhkBKSwsFQ6UqipHoWM2XgvYfx+GkmoifBI4osOlw6CE0dmcLyHitHEkGvLGdtIvpGmttx1rgnXZxZlZa1R801GTI33SsjyHDkL5ZC7rZEveCtM/eTE2h5Mco8k1HpIANQgn+YpIUqKGAsQ4rUKkmrhVmFXWRUKkfkua/A9flVGwH/seOAoDV5D71ZOaZzoFQ1xsI X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Mar 12, 2025 at 05:07:25PM +0000, Nikita Kalyazin wrote: > However if MISSING is not registered, the kernel will auto-populate with a > clear page, ie there is no way to inject custom content from userspace. To > explain my use case a bit more, the population thread will be trying to copy > all guest memory proactively, but there will inevitably be cases where a > page is accessed through pgtables _before_ it gets populated. It is not > desirable for such access to result in a clear page provided by the kernel. IMHO populating with a zero page in the page cache is fine. It needs to make sure all accesses will go via the pgtable, as discussed below in my previous email [1], then nobody will be able to see the zero page, not until someone updates the content then follow up with a CONTINUE to install the pgtable entry. If there is any way that the page can be accessed without the pgtable installation, minor faults won't work indeed. > > > as long as the content can only be accessed from the pgtable (either via > > mmap() or GUP on top of it), then afaiu it could work similarly like > > MISSING faults, because anything trying to access it will be trapped. [1] -- Peter Xu