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[192.41.114.227]) by smtp.gmail.com with ESMTPSA id z18-20020a1c4c12000000b003f423f5b659sm1555101wmf.10.2023.05.25.03.06.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 25 May 2023 03:06:42 -0700 (PDT) Date: Thu, 25 May 2023 11:06:40 +0100 From: Karim Manaouil To: npiggin@gmail.com Cc: david@redhat.com, akpm@linux-foundation.org, mike.kravetz@oracle.com, linux-mm@kvack.org, kmanaouilinux@gmail.com Subject: mm: Question: pte SMP data race in do_anomyous_page()? Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Rspamd-Queue-Id: C227B14001D X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: k44mio8j889q9utarecxnm9o7ana5t3n X-HE-Tag: 1685009204-108873 X-HE-Meta: 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 PNP0B/NZ 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.047225, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi, In do_anonymous_page(), a new page is allocated and zeroed, and the corresponding page struct is initialised (setting flags PageUptodate, PageSwapBacked, etc. and initialising the various counters). Then, set_pte_at() is called directly without calling smp_wmb() to make the updates above visible on other CPUs. This could race with a page table walker. The walker can read the new pte and try to access the page struct or the page content before the changes above were made visible. The reason I thought about this is because of the comment in pmd_install(), which describes exactly the same situation, so I wondered why the same thing is not considered in do_anonymous_page(). I might probably be missing something, but I would love to hear your comments. Cheers Karim