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a="189064686" X-IronPort-AV: E=Sophos;i="5.81,249,1610438400"; d="scan'208";a="189064686" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by orsmga103.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Mar 2021 16:22:04 -0700 IronPort-SDR: AWlq910Obb2IxrIuYWRhaxiVuzGnxGH21UbWid3C0rB3W0XnApXXzkxuFp+G67/R4YursDMbK/ VFiN4faswqIw== X-IronPort-AV: E=Sophos;i="5.81,249,1610438400"; d="scan'208";a="411653633" Received: from ngollapu-mobl1.amr.corp.intel.com (HELO [10.251.17.151]) ([10.251.17.151]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Mar 2021 16:22:04 -0700 Subject: Re: [PATCH v1 06/14] mm, x86: support the access bit on non-leaf PMD entries To: Yu Zhao , linux-mm@kvack.org Cc: Alex Shi , Andrew Morton , Dave Hansen , Hillf Danton , Johannes Weiner , Joonsoo Kim , Matthew Wilcox , Mel Gorman , Michal Hocko , Roman Gushchin , Vlastimil Babka , Wei Yang , Yang Shi , Ying Huang , linux-kernel@vger.kernel.org, page-reclaim@google.com References: <20210313075747.3781593-1-yuzhao@google.com> <20210313075747.3781593-7-yuzhao@google.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20210313075747.3781593-7-yuzhao@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 7BCD2407F8DB X-Stat-Signature: p5ysn6ywqep71wd3u7rjgychnr74gend Received-SPF: none (intel.com>: No applicable sender policy available) receiver=imf17; identity=mailfrom; envelope-from=""; helo=mga03.intel.com; client-ip=134.134.136.65 X-HE-DKIM-Result: none/none X-HE-Tag: 1615764133-834474 Content-Transfer-Encoding: quoted-printable 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 3/12/21 11:57 PM, Yu Zhao wrote: > Some architectures support the accessed bit on non-leaf PMD entries > (parents) in addition to leaf PTE entries (children) where pages are > mapped, e.g., x86_64 sets the accessed bit on a parent when using it > as part of linear-address translation [1]. Page table walkers who are > interested in the accessed bit on children can take advantage of this: > they do not need to search the children when the accessed bit is not > set on a parent, given that they have previously cleared the accessed > bit on this parent in addition to its children. I'd like to hear a *LOT* more about how this is going to be used. The one part of this which is entirely missing is the interaction with the TLB and mid-level paging structure caches. The CPU is pretty aggressive about setting no-leaf accessed bits when TLB entries are created. This *looks* to be depending on that behavior, but it would be nice to spell it out explicitly.