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[2003:cb:c700:2400:6b79:2aa:9602:7016]) by smtp.gmail.com with ESMTPSA id m9-20020a7bce09000000b003f049a42689sm36193382wmc.25.2023.05.02.12.05.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 May 2023 12:05:27 -0700 (PDT) Message-ID: <82368bae-b2cc-5d55-6de6-e3283cef063b@redhat.com> Date: Tue, 2 May 2023 21:05:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings Content-Language: en-US To: Peter Zijlstra Cc: Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Jason Gunthorpe , Jens Axboe , Matthew Wilcox , Dennis Dalessandro , Leon Romanovsky , Christian Benvenuti , Nelson Escobar , Bernard Metzler , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Ian Rogers , Adrian Hunter , Bjorn Topel , Magnus Karlsson , Maciej Fijalkowski , Jonathan Lemon , "David S . 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McKenney" , Christian Borntraeger , Mike Rapoport References: <1691115d-dba4-636b-d736-6a20359a67c3@redhat.com> <20230502172231.GH1597538@hirez.programming.kicks-ass.net> <406fd43a-a051-5fbe-6f66-a43f5e7e7573@redhat.com> <20230502185926.GE4253@hirez.programming.kicks-ass.net> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230502185926.GE4253@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02.05.23 20:59, Peter Zijlstra wrote: > On Tue, May 02, 2023 at 07:34:06PM +0200, David Hildenbrand wrote: >> Now, if we read folio->mapping after checking if the page we pinned is still >> mapped (PTE unchanged), at least the page we pinned cannot be reused in the >> meantime. I suspect that we can still read "NULL" on the second read. But >> whatever we dereference from the first read should still be valid, even if >> the second read would have returned NULL ("rcu freeing"). > > Right, but given it's the compiler adding loads we're not sure what if > anything it uses and it gets very hard to reason about the code. > > This is where READ_ONCE() helps, we instruct the compiler to only do a > single load and we can still reason about the code. I completely agree, and I think we should fix that in page_is_secretmem() as well. -- Thanks, David / dhildenb