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Thu, 27 Jul 2023 08:16:22 -0700 (PDT) MIME-Version: 1.0 References: <20230726214103.3261108-1-jannh@google.com> <31df93bd-4862-432c-8135-5595ffd2bd43@paulmck-laptop> In-Reply-To: From: Jann Horn Date: Thu, 27 Jul 2023 17:15:45 +0200 Message-ID: Subject: Re: [PATCH 0/2] fix vma->anon_vma check for per-VMA locking; fix anon_vma memory ordering To: Matthew Wilcox Cc: paulmck@kernel.org, Andrew Morton , Linus Torvalds , Peter Zijlstra , Suren Baghdasaryan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alan Stern , Andrea Parri , Will Deacon , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig , Joel Fernandes Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 0E3C31A000E X-Stat-Signature: yfhbix6aby8nox5roqmoh8gw3arr67jc X-HE-Tag: 1690470983-44502 X-HE-Meta: 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 D8VLOQL2 CP7emt1CPC9parcNB1vEumNlgLynsT3FioOwv09YtywaTYS0klMaICZxM4wXC1B8zGuCHQYgsUeY580cLvxZr4nyzTPm1VbqWQIS8adNu3Ppvs4k6cLs83RriI5IdNbNAgrKzB5gtypSnlG1d51dcvdBmtfDmX58FjzltmQMvq1k6PNWVswURxzCBAm7kSykPI6mGYUejVjowfPlvO68zHSqFAwueEVan89YPRDXaY/caTmKKw8E28rF0tpTeAlIkGBGKbtwJK3mqJWiOcuTv9ZEaQajrrahFKqQqVut9jO6woPynhJg0f6BKlSTYgwGTy0jf48zLPW29oUmhQPgwsG2D9qLGVyIWRvlo1MV0LSUJxq6mz4QuUMmsyMgPyZlqflt7/4KckYbdKsFhw4SxCpYFJQ== 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 Thu, Jul 27, 2023 at 5:07=E2=80=AFPM Matthew Wilcox wrote: > On Thu, Jul 27, 2023 at 04:39:34PM +0200, Jann Horn wrote: > > The other option is to replace the READ_ONCE() with a > > smp_load_acquire(), at which point it becomes a lot simpler to show > > that the code is correct. > > Aren't we straining at gnats here? The context of this is handling a > page fault, and we used to take an entire rwsem for read. I'm having > a hard time caring about "the extra expense" of an unnecessarily broad > barrier. > > Cost of an L3 cacheline miss is in the thousands of cycles. Cost of a > barrier is ... tens? Yeah, fair point. If it's hard to show correctness with READ_ONCE() we can just use smp_load_acquire() and call it a day.