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[2003:cb:c700:2400:6b79:2aa:9602:7016]) by smtp.gmail.com with ESMTPSA id u12-20020adfdd4c000000b0030635735a57sm2320577wrm.60.2023.05.02.05.53.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 May 2023 05:53:21 -0700 (PDT) Message-ID: <482f0c6e-ca5f-d0c2-1d99-ca26f70565df@redhat.com> Date: Tue, 2 May 2023 14:53:19 +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 v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings Content-Language: en-US To: Lorenzo Stoakes Cc: Peter Zijlstra , 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 . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Christian Brauner , Richard Cochran , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , linux-fsdevel@vger.kernel.org, linux-perf-users@vger.kernel.org, netdev@vger.kernel.org, bpf@vger.kernel.org, Oleg Nesterov , Jason Gunthorpe , John Hubbard , Jan Kara , "Kirill A . Shutemov" , Pavel Begunkov , Mika Penttila , Dave Chinner , Theodore Ts'o , Peter Xu , Paul McKenney References: <20230502111334.GP1597476@hirez.programming.kicks-ass.net> <20230502120810.GD1597538@hirez.programming.kicks-ass.net> <20230502124058.GB1597602@hirez.programming.kicks-ass.net> <4529b057-19ae-408b-8433-7d220f1871c0@lucifer.local> From: David Hildenbrand Organization: Red Hat In-Reply-To: <4529b057-19ae-408b-8433-7d220f1871c0@lucifer.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org On 02.05.23 14:52, Lorenzo Stoakes wrote: > On Tue, May 02, 2023 at 02:47:22PM +0200, David Hildenbrand wrote: >> On 02.05.23 14:40, Peter Zijlstra wrote: >>> On Tue, May 02, 2023 at 02:08:10PM +0200, Peter Zijlstra wrote: >>> >>>>>> >>>>>> >>>>>> if (folio_test_anon(folio)) >>>>>> return true; >>>>> >>>>> This relies on the mapping so belongs below the lockdep assert imo. >>>> >>>> Oh, right you are. >>>> >>>>>> >>>>>> /* >>>>>> * Having IRQs disabled (as per GUP-fast) also inhibits RCU >>>>>> * grace periods from making progress, IOW. they imply >>>>>> * rcu_read_lock(). >>>>>> */ >>>>>> lockdep_assert_irqs_disabled(); >>>>>> >>>>>> /* >>>>>> * Inodes and thus address_space are RCU freed and thus safe to >>>>>> * access at this point. >>>>>> */ >>>>>> mapping = folio_mapping(folio); >>>>>> if (mapping && shmem_mapping(mapping)) >>>>>> return true; >>>>>> >>>>>> return false; >>>>>> >>>>>>> +} >>> >>> So arguably you should do *one* READ_ONCE() load of mapping and >>> consistently use that, this means open-coding both folio_test_anon() and >>> folio_mapping(). >> >> Open-coding folio_test_anon() should not be required. We only care about >> PAGE_MAPPING_FLAGS stored alongside folio->mapping, that will stick around >> until the anon page was freed. >> > > Ack, good point! > >> @Lorenzo, you might also want to special-case hugetlb directly using >> folio_test_hugetlb(). >> > > I already am :) I guess you mean when I respin along these lines? Will port > that across to. Ha, I only stared at that reply, good! :) -- Thanks, David / dhildenb