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Howlett" , Lorenzo Stoakes , Michal Hocko , Paolo Bonzini , Sean Christopherson , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor mode Message-ID: References: <20251130111812.699259-1-rppt@kernel.org> <20251130111812.699259-5-rppt@kernel.org> <652578cc-eeff-4996-8c80-e26682a57e6d@amazon.com> <2d98c597-0789-4251-843d-bfe36de25bd2@kernel.org> <553c64e8-d224-4764-9057-84289257cac9@amazon.com> <76e3d5bf-df73-4293-84f6-0d6ddabd0fd7@amazon.com> <415a5956-1dec-4f10-be36-85f6d4d8f4b4@amazon.com> MIME-Version: 1.0 In-Reply-To: <415a5956-1dec-4f10-be36-85f6d4d8f4b4@amazon.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: U1TTdyj9sNVgYytlqKWflD4GeDCSz2aPz75mIlSgy_k_1764689772 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspam-User: X-Rspamd-Queue-Id: 90DCE40005 X-Rspamd-Server: rspam11 X-Stat-Signature: xezghnhbyi8wuq9dc1asw8s7uymanfk1 X-HE-Tag: 1764689776-756815 X-HE-Meta: U2FsdGVkX1+SgfV7VCfBzbnw3dGhZnhJ2o9sDUdiNsazPG+tuu8+X9ndvUKCUdU5w0l2KTiyoKWHNNTdtDHhoLlpDmFVGT/1E5gY2x2sGp/+FiRbNLBRMS172qWFHmwhUPHGVZiIoztQo7y5UIdFWGj8WTD41Q1xdsmn+XEsUKjwQ0RPsDr83yU8W2n4BkXnzb1Ps6ZnxClfRz4t9SE+OYYdEpomalaY708teMgM7eLa0/LmFoEMH8ZkhWu7S1CTlH1gS8mC05nAD/g7/jmq1kdRt/9NJaDLDuAKBHU1TqkaXDn//+vxelG5iMTUnf2+jstv2fNExykMBHDGltADrgWTdPn9AhHABxjEs/5U5tqY9MwvgKeV6uNiEAem9qqb//KwhEDNFWnY6hiJPDBCaJFsSEfXMQZ2URm7F11FxJWndAE7sEMhEtxAXJ/GsIFz/h1J1bifaFBUzwbbqOt0TZO5g2o8guJ6WF2mzHeXG/pyVDsoSTtQY8Mu07xLNkf2YFvK5WtXrEnswjrAi7t4+yKFm70ZGnHZeEfiDv762vPa6AbbDqGrPmPGQo8dccglawBhhbx0H19eJWBPdvSL5/+Jg8NVwpzF19zXq620RgPxg03UbJOIp6YYVsa7UsDLY9t2R+Hy1gIm4NqfNWE87eGWjpC5aZuzFNoZ81b4UWsuYg/Ra9SOeI7oKVwdALDWycngBS3ltNIRqE7WpXgc1NylNeBZlEhieR4Pk83cLGFehBp7vdSB+wEduY3g/g6xbR3VZ+4e5HpPaWvmRheckMsdDkDEPbm1TuYVIxpl/yQWjCsRrkugQLrETNq2Av2vYUqH5sMS9cvZcEraMBcKZV6VboyUFSXp5lQtnwDWaQtTtgBJ/QuBxr0V2WBQN54b2wwd9uRh6/FYkCij+RRJroBztdTp7plU8KCcytziIdOk/00Skqakwj8y1amkekm53Pxs6yEUBqx2ctbtmkN 61DSjz5w 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 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 Tue, Dec 02, 2025 at 11:50:31AM +0000, Nikita Kalyazin wrote: > > It looks fine indeed, but it looks slightly weird then, as you'll have two > > ways to populate the page cache. Logically here atomicity is indeed not > > needed when you trap both MISSING + MINOR. > > I reran the test based on the UFFDIO_COPY prototype I had using your series > [2], and UFFDIO_COPY is slower than write() to populate 512 MiB: 237 vs 202 > ms (+17%). Even though UFFDIO_COPY alone is functionally sufficient, I > would prefer to have an option to use write() where possible and only > falling back to UFFDIO_COPY for userspace faults to have better performance. Yes, write() should be fine. Especially to gmem, I guess write() support is needed when VMAs cannot be mapped at all in strict CoCo context, so it needs to be available one way or another. IIUC it's because UFFDIO_COPY (or memcpy(), I recall you used to test that instead) will involve pgtable operations. So I wonder if the VMA mapping the gmem will still be accessed at some point later (either private->share convertable ones for device DMAs for CoCo, or fully shared non-CoCo use case), then the pgtable overhead will happen later for a write()-styled fault resolution. >From that POV, above number makes sense. Thanks for the extra testing results. > > [2] > https://lore.kernel.org/all/7666ee96-6f09-4dc1-8cb2-002a2d2a29cf@amazon.com -- Peter Xu