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[2003:cb:c712:fa00:38eb:93ad:be38:d469]) by smtp.gmail.com with ESMTPSA id e27-20020a5d595b000000b003445bb2362esm10674889wri.65.2024.04.10.01.20.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Apr 2024 01:20:47 -0700 (PDT) Message-ID: <0e829c08-74f5-4487-97bd-4f81d8b70a2f@redhat.com> Date: Wed, 10 Apr 2024 10:20:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 04/18] mm: track mapcount of large folios in single value To: Zi Yan Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, cgroups@vger.kernel.org, linux-sh@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Andrew Morton , Matthew Wilcox , Peter Xu , Ryan Roberts , Yin Fengwei , Yang Shi , Jonathan Corbet , Hugh Dickins , Yoshinori Sato , Rich Felker , John Paul Adrian Glaubitz , Chris Zankel , Max Filippov , Muchun Song , Miaohe Lin , Naoya Horiguchi , Richard Chang References: <20240409192301.907377-1-david@redhat.com> <20240409192301.907377-5-david@redhat.com> <1DFEC99F-6BCE-4E6D-9EF2-267E8A94A705@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 232D8C0005 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: e43htak8j9xykm99e8nie87go1i4s7ma X-HE-Tag: 1712737252-545591 X-HE-Meta: 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 DwAU/ZRZ Na0oQQQ6VL+loRGjruKR/t+5+WbLduNCZIuoQTGB5rBBxXFQtnwhYeYh91vJc6cRW6/aWuU15zXiQsTd/8MLtFcDmXT8NqjJ4AwHNb+ZH1klaYrr56ppjRLbCqF1LL5Co1qvHSfF9n3zQx2iHnRQ0UucRrByf0/pick8X27nwE8mmpJPxH1w8MkVCkJmKesexC40qVq8p380vP54GU5CoK2oVCBph2UQs+LhGAIFLQuKPHDJmpp0hq6+me3F0LB33wq2N5+3NFtLD+g57QXhuLO8UpTGuHBKtNFVvbnA29RQSj6HcoPoJ+ni69r2hhIXsWzf8OcFpYq53o8v1Ifa89u2we5gcIvUHAl8qdo/Wz0hIZ0nP8ex3YK28mgMAV14nvafidIdrdOaMNNX6xe1LQsIpb9EdJ4ugEO7REZysv/oUOZI= 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: List-Subscribe: List-Unsubscribe: On 09.04.24 22:13, Zi Yan wrote: > On 9 Apr 2024, at 15:22, David Hildenbrand wrote: > >> Let's track the mapcount of large folios in a single value. The mapcount of >> a large folio currently corresponds to the sum of the entire mapcount and >> all page mapcounts. >> >> This sum is what we actually want to know in folio_mapcount() and it is >> also sufficient for implementing folio_mapped(). >> >> With PTE-mapped THP becoming more important and more widely used, we want >> to avoid looping over all pages of a folio just to obtain the mapcount >> of large folios. The comment "In the common case, avoid the loop when no >> pages mapped by PTE" in folio_total_mapcount() does no longer hold for >> mTHP that are always mapped by PTE. >> >> Further, we are planning on using folio_mapcount() more >> frequently, and might even want to remove page mapcounts for large >> folios in some kernel configs. Therefore, allow for reading the mapcount of >> large folios efficiently and atomically without looping over any pages. >> >> Maintain the mapcount also for hugetlb pages for simplicity. Use the new >> mapcount to implement folio_mapcount() and folio_mapped(). Make >> page_mapped() simply call folio_mapped(). We can now get rid of >> folio_large_is_mapped(). >> >> _nr_pages_mapped is now only used in rmap code and for debugging >> purposes. Keep folio_nr_pages_mapped() around, but document that its use >> should be limited to rmap internals and debugging purposes. >> >> This change implies one additional atomic add/sub whenever >> mapping/unmapping (parts of) a large folio. >> >> As we now batch RMAP operations for PTE-mapped THP during fork(), >> during unmap/zap, and when PTE-remapping a PMD-mapped THP, and we adjust >> the large mapcount for a PTE batch only once, the added overhead in the >> common case is small. Only when unmapping individual pages of a large folio >> (e.g., during COW), the overhead might be bigger in comparison, but it's >> essentially one additional atomic operation. >> >> Note that before the new mapcount would overflow, already our refcount >> would overflow: each mapping requires a folio reference. Extend the >> focumentation of folio_mapcount(). > > s/focumentation/documentation/ ;) Thanks! :) -- Cheers, David / dhildenb