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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id p1-20020a0c8c81000000b0063f8025407bsm3518958qvb.66.2023.08.09.14.23.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Aug 2023 14:23:58 -0700 (PDT) Date: Wed, 9 Aug 2023 17:23:46 -0400 From: Peter Xu To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, Andrew Morton , Jonathan Corbet , Mike Kravetz , Hugh Dickins , "Matthew Wilcox (Oracle)" , Ryan Roberts , Yin Fengwei , Yang Shi , Zi Yan Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios Message-ID: References: <20230809083256.699513-1-david@redhat.com> MIME-Version: 1.0 In-Reply-To: <20230809083256.699513-1-david@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Stat-Signature: wbfjjoqbz58c3rozxbgi3m97mepz84j8 X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 8C91720013 X-Rspam-User: X-HE-Tag: 1691616241-135538 X-HE-Meta: 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 /ECBybpl MTx76WKLLDkm7AxiCOSoo+81290V4eUb6kh0NKdDBeoc/kD5FtUxNsRpnrHI/9bVaalZnVC4CKBDATHKWaF0eVwIyXjWcdFb9uu0o0Ab9+CtdFZyn4b6tFBIu7ki8MRtIDGhf+NceIooKuBBptbM2bCUK5sYWD+XE+kIq1598FsAK3GQ3ZBfSB5U3rsiMmlSEgHWFV7SZz2a8avMEHIBui9lzNSHF23DIHLILWe+Exqik+NGkES8h99JxlbRDqsdZHDMKQpeS4Xr0tYTq/9cYf+QC1GFEx6ASkO7ib2qfEcLl8e48KMclTs7Aih4umeYXoRtEvIE7soSggWBcp6bf55+8QT17eLr6yN1Wi0Eg5Zi69jE= 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: Hi, David, Some pure questions below.. On Wed, Aug 09, 2023 at 10:32:56AM +0200, David Hildenbrand wrote: > Let's track the total mapcount for all large folios in the first subpage. > > The total mapcount is what we actually want to know in folio_mapcount() > and it is also sufficient for implementing folio_mapped(). This also > gets rid of any "raceiness" concerns as expressed in > folio_total_mapcount(). Any more information for that "raciness" described here? > > With sub-PMD THP becoming more important and things looking promising > that we will soon get support for such anon THP, we want to avoid looping > over all pages of a folio just to calculate the total mapcount. Further, > we may soon want to use the total mapcount in other context more > frequently, so prepare for reading it efficiently and atomically. Any (perhaps existing) discussion on reduced loops vs added atomic field/ops? I had a feeling that there's some discussion behind the proposal of this patch, if that's the case it'll be great to attach the link in the commit log. Thanks, -- Peter Xu