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[2003:d8:2f30:5a00:b30d:e6bc:74c3:d6f2]) by smtp.gmail.com with ESMTPSA id w12-20020a5d4b4c000000b0031434936f0dsm1171150wrs.68.2023.07.02.04.45.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 02 Jul 2023 04:45:48 -0700 (PDT) Message-ID: <6cec6f68-248e-63b4-5615-9e0f3f819a0a@redhat.com> Date: Sun, 2 Jul 2023 13:45:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.12.0 To: "Yin, Fengwei" , Zi Yan Cc: Matthew Wilcox , linux-mm@kvack.org, Vishal Moola , Hugh Dickins , Rik van Riel References: <7DCA075B-1E43-47B1-9402-66C54513D52E@nvidia.com> <310c4d8a-e14c-742b-5c6c-018c01ed897e@intel.com> <957ea888-a96b-89cc-29e2-973bb9e36f40@intel.com> From: David Hildenbrand Organization: Red Hat Subject: Re: Folio mapcount In-Reply-To: <957ea888-a96b-89cc-29e2-973bb9e36f40@intel.com> 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-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: D900B180009 X-Stat-Signature: pgq1ekyok7uq4sakmczzwn4dzdhh66gn X-HE-Tag: 1688298352-795202 X-HE-Meta: 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 c86LvwDQ 1Uc1q3qp7yG2yol9NEsRYFf3XYPe1pdqpoHWBltbJ1qd8xjJh+sz1LKFlyutRxX1Fnud4RZYkKB1E6+aYWinZtTIVn+y6WBCvR54MhAnrF2YLPHi08jeJPmrQ+fswoADF97pGIvjCXDCZWXPBsK6jjs6ErlEDjuYh4FPZhGx5IvPUZGav6viG9CfW8rFnxct5Pb3XP9jg+WBv/205ejebf+rp7xfoKPgYDl2W+oVaNIQ02fbZiWaThj5SgvHEGcvYdyolu0FkdibdMWrwGv5ElJBhx2F+NHGCiJqc3XXRvuQN/Jizacfx0yZbY7KcLCBSK3qhEQRNOmkYd5wmgOwTR/WzFXRwATXgveFpi1+dp0CnSMiwZ+hiWAd4xf6LkOzlfnkeNvOh2Qu5FmxhDRqvHs18kWnLP9WIBDGtkvy0cl6Dqan42dHN7mNWIB0XQFC3jHkuboxWvQscryE= 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 02.07.23 11:50, Yin, Fengwei wrote: > > > On 7/1/2023 9:17 AM, Zi Yan wrote: >> In kernel, almost all code only cares: 1) if a page/folio has extra pins >> by checking if mapcount is equal to refcount + extra, and 2) >> if a page/folio is mapped multiple times. A single mapcount can meet >> these two needs. > For 2, how can we know whether a page/folio is mapped multiple times from > single mapcount? My understanding is we need two counts as folio could be > partial mapped. Yes, a single mapcount is most probably insufficient. I started analyzing all existing users and use cases, trying to avoid walking page tables. If we want to get rid of all of (most) sub-page mapcounts, we'd probably want: (1) Total mapcount (compound + any sub-page): page_mapped(), pagecount vs. refcount games, ... (2) Compound mapcount (for PMD/PUD-mappale THP only): (2) - (1) tells you if it's only PMD mapped or also PTE-mapped. For example, for statistics but also swapout code. (3) Mapcount of first (or any other) subpage (compount+subpage): for folio_estimated_sharers(). For anon pages, I'm thinking about remembering an additional (1) Page/folio creator (MM pointer/identification) (2) Page/folio creator mapcount When optimizing a PTE-mapped THP (especially not- pmd-mappale) for the fork()+exec() case, we'd have to walk page tables to see if all folio references come from this MM. The page/folio creator exactly avoids that completely. We might need a mechanism to synchronize against mapping/unmapping of this folio from the creator concurrently (relevant when mapped into multiple page tables). Further, for (1) we'd want a 64bit mapcount for large folios, which implies a 64bit refcount. For smallish folios, we don't really care. We should most probably use a bi-weekly MM meeting to discuss that. Hopefully, I have a full understanding of all use cases and requirements until then. Don't have sufficient time to look into all the nasty details right now. -- Cheers, David / dhildenb