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[209.85.167.46]) by smtp.gmail.com with ESMTPSA id w202sm2144168lff.182.2021.01.08.11.42.55 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 08 Jan 2021 11:42:55 -0800 (PST) Received: by mail-lf1-f46.google.com with SMTP id o19so25698803lfo.1 for ; Fri, 08 Jan 2021 11:42:55 -0800 (PST) X-Received: by 2002:a2e:6f17:: with SMTP id k23mr2233926ljc.411.1610134974839; Fri, 08 Jan 2021 11:42:54 -0800 (PST) MIME-Version: 1.0 References: <20210108171517.5290-1-will@kernel.org> In-Reply-To: From: Linus Torvalds Date: Fri, 8 Jan 2021 11:42:39 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 0/3] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag To: Will Deacon Cc: Linux Kernel Mailing List , Linux-MM , Linux ARM , Catalin Marinas , Jan Kara , Minchan Kim , Andrew Morton , "Kirill A . Shutemov" , Vinayak Menon , Hugh Dickins , Android Kernel Team Content-Type: text/plain; charset="UTF-8" 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 Fri, Jan 8, 2021 at 11:34 AM Linus Torvalds wrote: > > Yeah, I think that's a side effect of "now the code really makes a lot > more sense". Your subsequent patches 2-3 certainly are much simpler > now On that note - they could be simpler still if this was just done entirely unconditionally.. I'm taking your word for "it makes sense", but when you say On CPUs with hardware AF/DBM, initialising prefaulted PTEs as 'old' improves vmscan behaviour and does not appear to introduce any overhead. in the description for patch 3, it makes me wonder how noticeable the overhead is on the hardware that _does_ take a fault on old pte's.. IOW, it would be lovely to see numbers if you have any like that.. Both ways, actually. Because I also wonder how noticeable the vmscan improvement is. You say there's no measurable overhead for platforms with hardware dirty/accessed bits, but maybe there's not a lot of measurable improvements from a more exact accessed bit either? Linus