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[2003:cb:c707:5c00:e650:bcd7:e2a0:54fe]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b36863962sm19381811f8f.108.2024.08.01.06.15.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 01 Aug 2024 06:15:33 -0700 (PDT) Message-ID: <3636af63-8865-4ef5-a2f3-2a6538aca873@redhat.com> Date: Thu, 1 Aug 2024 15:15:31 +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: Ryan Roberts , Dev Jain , akpm@linux-foundation.org, willy@infradead.org Cc: 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> <3b82e195-5871-4880-9ce5-d01bb751f471@redhat.com> <738342dc-4a87-4dcc-a515-a9dc085e3186@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 9413pfxf5c9o59e1876fudfr9rr1zetd X-Rspamd-Queue-Id: F28B2180003 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1722518137-696050 X-HE-Meta: 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 c6fuIth4 X9gjXh1IPAVc3SAHAAcnkbzY5tcSB2JyqMs3pH5WM0TY9+CyMMhSUWtPzY04X87Tqlkms9QuCfozPxZKd6Fs99h8byrwJPK1aDXh0tpkybTzNNpGopf4JQU/eQzPnDbJLSkjToJWHxaWG13eRoPriSeOolyz6o2wOO+tM+Us19UHqX+jPm9nymwTuSPwxkOoOPh/VVDraPjReU1TXSIvhr+IQ2K+JwCUiSwRXza6CYXNIdWFIqqIdo7B5jeNTsrFIV9BewVe7nZB0hLDthiXprOiv0hin/i915dGJMxgTpbJEawYPFwNDZv0TwiuxxwVUPm5Xk7d4h2KfSmMlo9zXQQGMKNbkGTB6iIl5TlImOUqjjRKKEc6jFg/bpA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000082, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> What I am still missing is why this is (a) arm64 only; and (b) if this is >> something we should really worry about. There are other reasons (e.g., >> speculative references) why migration could temporarily fail, does it happen >> that often that it is really something we have to worry about? > > The test fails consistently on arm64. It's my rough understanding that it's > failing due to migration backing off because the fault handler has raised the > ref count? (Dev correct me if I'm wrong). > > So the real question is, is it a valid test in the first place? Should we just > delete the test or do we need to strengthen the kernel's guarrantees around > migration success? I think the test should retry migration a number of times in case it fails. But if it is a persistent migration failure, the test should fail. -- Cheers, David / dhildenb