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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pPXVC-000fbk-P5; Tue, 07 Feb 2023 23:35:54 +0000 Date: Tue, 7 Feb 2023 23:35:54 +0000 From: Matthew Wilcox To: James Houghton Cc: linux-mm@kvack.org, Vishal Moola , Hugh Dickins , Rik van Riel , David Hildenbrand , "Yin, Fengwei" Subject: Re: Folio mapcount Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: 18D9540002 X-Rspamd-Server: rspam01 X-Stat-Signature: fgq9sntwayrw3wjggssmq54qeybacoi7 X-HE-Tag: 1675812957-128019 X-HE-Meta: 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 t7q2BsvU MM+ehUcJ86rQaj1gETWI2BmXyoDau5HZcbvE4XyU0gDHOS1doPH8YEuht6Sf9vw3lq4v81Ttsi4hfZLkFio9rMUqUUZ7zoGPRygUNxP3BYM/dMTdSmlLJ8AVao+u+id22uSEp1/uZYDiMY23SaadNfKgR0ETWB4NVhZvJtYon0WYVjG/AM+06R6107t/SQrEvJftmDRAGmWGHMhTqXeyFcI7u0RmO6h7M5mPd 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 Tue, Feb 07, 2023 at 03:27:07PM -0800, James Houghton wrote: > So page_vma_mapped_walk() might have to walk up to HPAGE_PMD_NR-ish > PTEs (if we find a bunch of pte_none() PTEs). Just curious, could that > be any slower than what we currently do (like, incrementing up to > HPAGE_PMD_NR-ish subpage mapcounts)? Or is it not a concern? I think it's faster. Both of these operations work on folio_nr_pages() entries ... but a page table is 8 bytes and a struct page is 64 bytes. >From a CPU prefetching point of view, they're both linear scans, but PTEs are 8 times denser. The other factor to consider is how often we do each of these operations. Mapping a folio happens ~once per call to mmap() (even though it's delayed until page fault time). Querying folio_total_mapcount() happens ... less often, I think? Both are going to be quite rare since generally we map the entire folio at once.