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(p200300cbc70c6c0092a406f8ff7e6853.dip0.t-ipconnect.de. [2003:cb:c70c:6c00:92a4:6f8:ff7e:6853]) by smtp.gmail.com with ESMTPSA id hn8-20020a05600ca38800b003fee6e170f9sm19270240wmb.45.2023.09.06.02.42.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 06 Sep 2023 02:42:35 -0700 (PDT) Message-ID: <0240468f-3cc5-157b-9b10-f0cd7979daf0@redhat.com> Date: Wed, 6 Sep 2023 11:42:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 To: Christoph Hellwig , Jan Kara , David Howells , Peter Xu Cc: Lei Huang , miklos@szeredi.hu, Xiubo Li , Ilya Dryomov , Jeff Layton , Trond Myklebust , Anna Schumaker , Latchesar Ionkov , Dominique Martinet , Christian Schoenebeck , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , John Fastabend , Jakub Sitnicki , Boris Pismenny , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org, linux-mm@kvack.org, v9fs@lists.linux.dev, netdev@vger.kernel.org References: <20230905141604.GA27370@lst.de> From: David Hildenbrand Organization: Red Hat Subject: Re: getting rid of the last memory modifitions through gup(FOLL_GET) In-Reply-To: <20230905141604.GA27370@lst.de> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 29AE4140005 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: 9m6y1zerk9whjqiohxge5oq34yoqc8ss X-HE-Tag: 1693993360-633410 X-HE-Meta: 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 yvkJDD+n 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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: On 05.09.23 16:16, Christoph Hellwig wrote: > Hi all, Hi, > > we've made some nice progress on converting code that modifies user > memory to the pin_user_pages interface, especially though the work > from David Howells on iov_iter_extract_pages. This thread tries to > coordinate on how to finish off this work. > > The obvious next step is the remaining users of iov_iter_get_pages2 > and iov_iter_get_pages_alloc2. We have three file system direct I/O > users of those left: ceph, fuse and nfs. Lei Huang has sent patches > to convert fuse to iov_iter_extract_pages which I'd love to see merged, > and we'd need equivalent work for ceph and nfs. > > The non-file system uses are in the vmsplice code, which only reads vmsplice really has to be fixed to specify FOLL_PIN|FOLL_LONGTERM for good; I recall that David Howells had patches for that at one point. (at least to use FOLL_PIN) > from the pages (but would still benefit from an iov_iter_extract_pages > conversion), and in net. Out of the users in net, all but the 9p code > appear to be for reads from memory, so they don't pin even if a > conversion would be nice to retire iov_iter_get_pages* APIs. > > After that we might have to do an audit of the raw get_user_pages APIs, > but there probably aren't many that modify file backed memory. ptrace should apply that ends up doing a FOLL_GET|FOLL_WRITE. Further, KVM ends up using FOLL_GET|FOLL_WRITE to populate the second-level page tables for VMs, and uses MMU notifiers to synchronize the second-level page tables with process page table changes. So once a PTE goes from writable -> r/o in the process page table, the second level page tables for the VM will get updated. Such MMU users are quite different from ordinary GUP users. Converting the latter to FOLL_PIN is not desired (as it would implicitly trigger COW-unsharing on KSM pages -- but GUP+MMU notifiers is different to ordinary GUP+read/write where there is no such synchronization). Converting ptrace might not be desired/required as well (the reference is dropped immediately after the read/write access). The end goal as discussed a couple of times would be the to limit FOLL_GET in general only to a couple of users that can be audited and keep using it for a good reason. Arbitrary drivers that perform DMA should stop using it (and ideally be prevented from using it) and switch to FOLL_PIN. -- Cheers, David / dhildenb