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(p200300cbc713d600afc54312176f3fb0.dip0.t-ipconnect.de. [2003:cb:c713:d600:afc5:4312:176f:3fb0]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-441b2aed5e8sm90516285e9.16.2025.05.02.06.48.07 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 May 2025 06:48:07 -0700 (PDT) Message-ID: <63d93fc8-874f-4620-8df3-160cb24edc0c@redhat.com> Date: Fri, 2 May 2025 15:48:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: MSG_ZEROCOPY and the O_DIRECT vs fork() race To: David Howells , Andrew Lunn Cc: John Hubbard , "David S. Miller" , Jakub Kicinski , willy@infradead.org, netdev@vger.kernel.org, linux-mm@kvack.org References: <0aa1b4a2-47b2-40a4-ae14-ce2dd457a1f7@lunn.ch> <1015189.1746187621@warthog.procyon.org.uk> <1021352.1746193306@warthog.procyon.org.uk> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <1021352.1746193306@warthog.procyon.org.uk> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 9TpS_rdXzgoUWoquddMXgx4mLcM66hseyx10iCLqrmU_1746193689 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 2F6BA180010 X-Stat-Signature: 8arfk88mhmzpe1ednu1xjjudb4tqq9uh X-Rspam-User: X-HE-Tag: 1746193692-448442 X-HE-Meta: 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 wft3RKeL fzusrgDyRkjx3WyCiDFdzG26nVczT5aLLJwg9EsFKbXHjcC7wPz+8QtbPh/Yc7bJ/hL9gXlZouoVimjFyY3m4mYkoClqty/54zdeb7WQ1r9W3sve1BqNmupYatZXnDMZpu3fL9II/Uda1vDI12HAHDtO29W5QNhNqJWhSHnI1pgDXKzH0GCbUj/Nmqodop/ZIqPlLXkVge/QtBlq1/3K0hwfH0ofYECapBqY3YRLwLebQTKtjEpST5tCk5LMC/RteWjW31CA2PifjX+1weAJ+5u0qO23wbOAMVhAFcWpsVUJb38s85+ah6BPweiaGnDJCVh+68wlpMS4H97mIJaBG15ON025NOCFbQH6f3yJrID9E7rg1+aLdEEuWZyK1nOxGmGMUTwyRJCOavOatCDZ7l10tJcPEcOsWT8i5LGdn9Q9QBdWO4Ph4K3ZbnuAOUdPfvE4f 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 02.05.25 15:41, David Howells wrote: > Andrew Lunn wrote: > >>> I'm looking into making the sendmsg() code properly handle the 'DIO vs >>> fork' issue (where pages need pinning rather than refs taken) and also >>> getting rid of the taking of refs entirely as the page refcount is going >>> to go away in the relatively near future. >> >> Sorry, new to this conversation, and i don't know what you mean by DIO >> vs fork. > > As I understand it, there's a race between O_DIRECT I/O and fork whereby if > you, say, start a DIO read operation on a page and then fork, the target page > gets attached to child and a copy made for the parent (because the refcount is > elevated by the I/O) - and so only the child sees the result. This is made > more interesting by such as AIO where the parent gets the completion > notification, but not the data. > > Further, a DIO write is then alterable by the child if the DMA has not yet > happened. > > One of the things mm/gup.c does is to work around this issue... However, I > don't think that MSG_ZEROCOPY handles this - and so zerocopy sendmsg is, I > think, subject to the same race. If it's using FOLL_PIN it works. If not, it's still to be fixed. -- Cheers, David / dhildenb