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[2003:cb:c71a:8a00:8200:f041:4b87:a8be]) by smtp.gmail.com with ESMTPSA id l13-20020adfe9cd000000b0031934b035d2sm2586529wrn.52.2023.08.10.15.27.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Aug 2023 15:27:14 -0700 (PDT) Message-ID: <8222bf8f-6b99-58f4-92cc-44113b151d14@redhat.com> Date: Fri, 11 Aug 2023 00:27:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 To: Matthew Wilcox , Peter Xu Cc: Ryan Roberts , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, Andrew Morton , Jonathan Corbet , Mike Kravetz , Hugh Dickins , Yin Fengwei , Yang Shi , Zi Yan References: <20230809083256.699513-1-david@redhat.com> <155bd03e-b75c-4d2d-a89d-a12271ada71b@arm.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: ABFFE2001B X-Stat-Signature: 7mwxgsiob4gqayxkdzk8e8tkt5n6fac1 X-Rspam-User: X-HE-Tag: 1691706438-852979 X-HE-Meta: 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 wOA0CuO/ IWdyrc7NYXNs5Bp+jA7h9+BW+CxLOXhRO63fAXp2UInQrl6r/r8StRDlPF610pNoDRWxT3XmnHxBQuWaiGAFOEmzMSiej/EJiYeLkXq9og4qEpzB83JD9BXibCF8P3SYmgdOs2bbwK8nEBaXGmzNsRDGN4QNABozvlbUR2iMaMkEG1Z4vwkmjtk0eZvttt2nVDCVdcrTADRjfuXGkGjoogweylNLrtYsZxgc2AKTXj9bJs0U6DUXNItXv5MkMexun88dSmQYVDRmBgRCNNhhe7XF57rmcq0ZPTOd+6nZ1gxMM62ZmD8pQUNYRx8MMEgLv+eSxNitQcYLRqc8IhOLk180yocBACkFELx0fklKeZMFJucFGJc2Bor/ezexXKwn5AReT 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 10.08.23 23:48, Matthew Wilcox wrote: > On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote: >> AFAICS if that patch was all correct (while I'm not yet sure..), you can >> actually fit your new total mapcount field into page 1 so even avoid the >> extra cacheline access. You can have a look: the trick is refcount for >> tail page 1 is still seems to be free on 32 bits (if that was your worry >> before). Then it'll be very nice if to keep Hugh's counter all in tail 1. > > No, refcount must be 0 on all tail pages. We rely on this in many places > in the MM. Very right. One could theoretically 1) Move the compound/entire mapcount to page[2] 2) Make hugetlb stop using the entire mapcount and only the total mapcount. 3) Then leave total_mapcount and nr_pages_mapped in page[1] 4) Make page_mapcount() use total_mapcount for hugetlb. When (un)mapping a PMD-mapped THP, we would go to page[2]. Otherwise, only page[1]. The entire mapcount, similarly to nr_pages_mapped, primarily serves to get memory stats right; well, and to implement page_mapcount() for THP. But I'm not 100% sure yet if the overhead from having nr_pages_mapped in page[2] is significant enough at this point. -- Cheers, David / dhildenb