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[209.85.160.54]) by smtp.gmail.com with ESMTPSA id l75-20020a9d1b51000000b00660d73c8bdesm7240883otl.50.2022.10.19.10.41.06 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 Oct 2022 10:41:06 -0700 (PDT) Received: by mail-oa1-f54.google.com with SMTP id 586e51a60fabf-12c8312131fso21533598fac.4 for ; Wed, 19 Oct 2022 10:41:06 -0700 (PDT) X-Received: by 2002:a05:6870:c0c9:b0:127:c4df:5b50 with SMTP id e9-20020a056870c0c900b00127c4df5b50mr6076270oad.126.1666201256175; Wed, 19 Oct 2022 10:40:56 -0700 (PDT) MIME-Version: 1.0 References: <20220815071332.627393-1-yuzhao@google.com> <20220815071332.627393-9-yuzhao@google.com> In-Reply-To: From: Linus Torvalds Date: Wed, 19 Oct 2022 10:40:40 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v14 08/14] mm: multi-gen LRU: support page table walks To: Yu Zhao Cc: Peter Zijlstra , Andrew Morton , Andi Kleen , Aneesh Kumar , Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Johannes Weiner , Jonathan Corbet , Matthew Wilcox , Mel Gorman , Michael Larabel , Michal Hocko , Mike Rapoport , Tejun Heo , Vlastimil Babka , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, page-reclaim@google.com, Brian Geffon , Jan Alexander Steffens , Oleksandr Natalenko , Steven Barrett , Suleiman Souhlal , Daniel Byrne , Donald Carr , =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= , Konstantin Kharlamov , Shuang Zhai , Sofia Trinh , Vaibhav Jain Content-Type: text/plain; charset="UTF-8" ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1666201268; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=o7oESz5tomQbWcoWEgo0/iqd576fd8JG2PTmQCWKHUY=; b=2xq8JivqEQcXzo9zwol3P5xSh9SRpgLNN3aGlU6g9sKyJmTNXzjujjSudPlaTzFMiT/L6z doMb3JWpUPsosunFsh+AOpG4Ay59USfZhr2B64f5xxgfw5eXW4sk5BUoHL8CGgBOBJXLX7 vIdgEq0wXH4OJdSL0z33cEElzYIsxWI= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=LhnaCxQk; spf=pass (imf19.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.167.172 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1666201268; a=rsa-sha256; cv=none; b=SQUkHSbQ+kZHbqgRNcXbB9vbJPnqm7HwHYHd7H70zfdejdOajqE1Pw4Z+oH1eh9gWt8NRJ 1R/1uR2m6O69cd6MaKqqp9CP3ePfqs5DYtrKUA1wi55buhOYYxlvdNMt8lugfg7jUO/BRY tihpivRyJSfNi5VjxB/x6OaOPtDQwKs= Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=google header.b=LhnaCxQk; spf=pass (imf19.hostedemail.com: domain of torvalds@linuxfoundation.org designates 209.85.167.172 as permitted sender) smtp.mailfrom=torvalds@linuxfoundation.org; dmarc=none X-Stat-Signature: hwab4sxybhycrut3teet8j1hjdd4a6bi X-Rspamd-Queue-Id: 325271A0039 X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1666201267-827863 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 Tue, Oct 18, 2022 at 10:51 PM Yu Zhao wrote: > > pmd_read_atomic() should have a built-in READ_ONCE() in the first > place. I really think that is the right thing to do. Let's just move the barrier in *there* instead. It really should use 'READ_ONCE()', but it sadly cannot do that portably, because 'pmd_t' may be a multi-word structure. Of course, the x86-32 code does this all *almost* right, and implements its own version of pmd_read_atomic(), but then sadly does it _without_ actually using READ_ONCE() there. So even when we could do it right, we don't. But the x86-32 implementation of pmd_read_atomic() would be trivial to fix to just use READ_ATOMIC, and the generic implementation should just have a "barrier()" in it so that we wouldn't need crazy barriers in the users. Because as you say, the function is already called "read_atomic", and it should damn well *act* that way then. Hmm? Linus