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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id f5-20020a0cf3c5000000b0065cfec43097sm4215956qvm.39.2023.10.02.08.21.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Oct 2023 08:21:23 -0700 (PDT) Date: Mon, 2 Oct 2023 11:21:20 -0400 From: Peter Xu To: David Hildenbrand Cc: Jann Horn , Suren Baghdasaryan , akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, hughd@google.com, mhocko@suse.com, axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, Liam.Howlett@oracle.com, zhangpeng362@huawei.com, bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI Message-ID: References: <20230923013148.1390521-1-surenb@google.com> <20230923013148.1390521-3-surenb@google.com> <03f95e90-82bd-6ee2-7c0d-d4dc5d3e15ee@redhat.com> <98b21e78-a90d-8b54-3659-e9b890be094f@redhat.com> <85e5390c-660c-ef9e-b415-00ee71bc5cbf@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: 6kz9n1gwfcouufwzpmp88kjd5xw3mmmo X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: D07FBA0033 X-Rspam-User: X-HE-Tag: 1696260096-57545 X-HE-Meta: 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 KkVnVrWl dB3J0w1bOd1S/8thBRZM2ktmwH7RDUX6kiZ4dTqBVLqsBS9vWUXWD0YoFOD7jW0XaYokq2b3+pKjuek6ooKhboFrmUwb9t3gAGfEsz2xwAeuMRCj0TGnd2N6NxvF37UfbmXna5fYqkR5D2DlaMyu8im9OGC4b9GBOS+Y+6Xbz04II7rf3FxTOhkmCvIbc5zl/Z+h6MTdXu+QlF828k/R2i3vySk8bhZrX69u9Q7FFhywtz/RzjiDpZIya6fj/Ra1QGrs6Kq3JctdWEfmFgCQlHrVyKHgqoNQzHrLhYise4TYmQzqA4uokCRCfZpgNOxXncECbTqorZv46GQRyvvTLvOT21rUHdYBBQzdrBgdcYHHoHJttwgCNG12N0J7Q3BTHjZOHfhnIm8pVZVkt93+qIbNsv04Z1Jmehp0Q X-Bogosity: Ham, tests=bogofilter, spamicity=0.000049, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Oct 02, 2023 at 10:00:03AM +0200, David Hildenbrand wrote: > In case we cannot simply remap the page, the fallback sequence (from the > cover letter) would be triggered. > > 1) UFFDIO_COPY > 2) MADV_DONTNEED > > So we would just handle the operation internally without a fallback. Note that I think there will be a slight difference on whole remap atomicity, on what happens if the page is modified after UFFDIO_COPY but before DONTNEED. UFFDIO_REMAP guarantees full atomicity when moving the page, IOW, threads can be updating the pages when ioctl(UFFDIO_REMAP), data won't get lost during movement, and it will generate a missing event after moved, with latest data showing up on dest. I'm not sure that means such a fallback is a problem, Suren may know better with the use case. Thanks, -- Peter Xu