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[2003:cb:c705:d100:871b:ec55:67d:5247]) by smtp.gmail.com with ESMTPSA id r18-20020adfce92000000b0031272fced4dsm15929196wrn.52.2023.08.01.05.28.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 01 Aug 2023 05:28:31 -0700 (PDT) Message-ID: <3427735b-2a73-2df7-ebd9-0d1066a55771@redhat.com> Date: Tue, 1 Aug 2023 14:28:30 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Subject: Re: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages) To: Jason Gunthorpe Cc: "Kasireddy, Vivek" , "Kim, Dongwon" , "Chang, Junxiao" , "dri-devel@lists.freedesktop.org" , Alistair Popple , Hugh Dickins , Peter Xu , "linux-mm@kvack.org" , Gerd Hoffmann , Mike Kravetz References: <48f22686-2c1b-fd9d-91ba-da6105d410db@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: 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: 975DA40009 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 498ca4kgshygsn6pw4drutdacdk17p85 X-HE-Tag: 1690892915-877772 X-HE-Meta: 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 UOYCbKLL HB7AnvqibaH0UBnzYDNiDozTutsj4MRQLPO7KKi8cXLBib5LcM4ApQzTldsowYMP4NfHHWCy3Rmdoza9n/JGB9CeFAOp7Gl8anNIWIlm8K20GwiQ8SsJd1QiLcJt+yjqaTOHRzeIIwELjfurmkfUWSm3HfHwtrjRP5G8i78D0QVHoCC8wwMWgo8DQZqtJSGInhNUZEzXLJZDABKtQKoKpn7eJNC+qcvPMHk3gcix8GlyhHrcKwcA7wk5L37uWG5FwkkY0Ry4a6L2I8OEtw57010dNNpfO92TtKD0UEuWwil5qnKso6ECwI6S51/EBTShoft1K2fOb7mcx4/NyfRwFedk6Td3cnzniupb4Bp3KsGvQRwnlSRmaFHywcQXFGn5fznqld6gL/Yn22CyAcpgeidULCSD/qiDy0AAHR3n9SVJjzIbybjJKNJRMfLCIShn0Nh49JsSIpbYGwj6rAnwYLiLxwg== 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 01.08.23 14:26, Jason Gunthorpe wrote: > On Tue, Aug 01, 2023 at 02:26:03PM +0200, David Hildenbrand wrote: >> On 01.08.23 14:23, Jason Gunthorpe wrote: >>> On Tue, Aug 01, 2023 at 02:22:12PM +0200, David Hildenbrand wrote: >>>> On 01.08.23 14:19, Jason Gunthorpe wrote: >>>>> On Tue, Aug 01, 2023 at 05:32:38AM +0000, Kasireddy, Vivek wrote: >>>>> >>>>>>> You get another invalidate because the memfd removes the zero pages >>>>>>> that hmm_range_fault installed in the PTEs before replacing them with >>>>>>> actual writable pages. Then you do the move, and another >>>>>>> hmm_range_fault, and basically the whole thing over again. Except this >>>>>>> time instead of returning zero pages it returns actual writable >>>>>>> page. >>>>> >>>>>> Ok, when I tested earlier (by registering an invalidate callback) but without >>>>>> hmm_range_fault(), I did not find this additional invalidate getting triggered. >>>>>> Let me try with hmm_range_fault() and see if everything works as expected. >>>>>> Thank you for your help. >>>>> >>>>> If you do not get an invalidate then there is a pretty serious bug in >>>>> the mm that needs fixing. >>>>> >>>>> Anything hmm_range_fault() returns must be invalidated if the >>>>> underying CPU mapping changes for any reasons. Since hmm_range_fault() >>>>> will populate zero pages when reading from a hole in a memfd, it must >>>>> also get an invalidation when the zero pages are changed into writable >>>>> pages. >>>> >>>> Can you point me at the code that returns that (shared) zero page? >>> >>> It calls handle_mm_fault() - shouldn't that do it? Same as if the CPU >>> read faulted the page? >> >> To the best of my knowledge, the shared zeropage is only used in >> MAP_PRIVATE|MAP_AON mappings and in weird DAX mappings. >> >> If that changed, we have to fix FOLL_PIN|FOLL_LONGTERM for MAP_SHARED VMAs. >> >> If you read-fault on a memfd hole, you should get a proper "zeroed" >> pagecache page that effectively "filled that hole" -- so there is no file >> hole anymore. > > Sounds fine then :) Right, the "the zero pages are changed into writable pages" in your above comment just might not apply, because there won't be any page replacement (hopefully :) ). -- Cheers, David / dhildenb