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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b367fce64sm18847318f8f.61.2024.08.01.01.42.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 01:42:06 -0700 (PDT) Message-ID: <3b82e195-5871-4880-9ce5-d01bb751f471@redhat.com> Date: Thu, 1 Aug 2024 10:42:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Race condition observed between page migration and page fault handling on arm64 machines To: Dev Jain , akpm@linux-foundation.org, willy@infradead.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, osalvador@suse.de, baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, ioworker0@gmail.com, gshan@redhat.com, mark.rutland@arm.com, kirill.shutemov@linux.intel.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, broonie@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20240801081657.1386743-1-dev.jain@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 379C840012 X-Stat-Signature: mp85t3rdo6mwaoe19madgntmexbj1wra X-HE-Tag: 1722501731-756241 X-HE-Meta: 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 xWXrmAdA dX1NVGoNAAt2/KXtq8gE3sMC62t1ZVadX/md0iLypGBTztVQ16lw+iuubzvSCQKVyjOGQnM+k4fUZ85w4BfeU1ZsVL+8BhGu76wWnefCx5baqe4CeveZmJ09d7UUCcbckszlRB9SRvnTw68YzRPDFBPAWOBmBaQ40/DIyZWZtbAX7m0epq+JkOf08yFHlIgmyR6ObrVqvEsjvTfAWITYK61RDFDyrZRPwZ0LAk66dHOTXsVHrBUiWMKNmSjWl0EUpW9fJPO611GVTFLw9QXeo+/NdEftCo0/96kYgA/bOHwh7AYYteWEu1Ui1rjcO5LZAgWnHLSLBPdwxjeq7VYOm3AsWhLnywq9ZPLIqdgWxph6zEm/CdCFxBm26ow== 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 01.08.24 10:16, Dev Jain wrote: > I and Ryan had a discussion and we thought it would be best to get feedback > from the community. > > The migration mm selftest currently fails on arm64 for shared anon mappings, > due to the following race: Do you mean MAP_SHARED|MAP_ANON or MAP_PRIVATE|MAP_ANON_fork? Because you note shmem below, I assume you mean MAP_SHARED|MAP_ANON > > Migration: Page fault: > try_to_migrate_one(): handle_pte_fault(): > 1. Nuke the PTE PTE has been deleted => do_pte_missing() > 2. Mark the PTE for migration PTE has not been deleted but is just not "present" => do_swap_page() > In filemap_fault_recheck_pte_none() we recheck under PTL to make sure that a temporary pte_none() really was persistent pte_none() and not a temporary pte_none() under PTL. Should we do something similar in do_fault()? I see that we already do something like that on the "!vma->vm_ops->fault" path. But of course, there is a tradeoff between letting migration (temporarily) fail and grabbing the PTL during page faults. -- Cheers, David / dhildenb