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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1qFw9w-007MkN-Cw; Sun, 02 Jul 2023 12:26:32 +0000 Date: Sun, 2 Jul 2023 13:26:32 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: "Yin, Fengwei" , Zi Yan , linux-mm@kvack.org, Vishal Moola , Hugh Dickins , Rik van Riel Subject: Re: Folio mapcount Message-ID: References: <7DCA075B-1E43-47B1-9402-66C54513D52E@nvidia.com> <310c4d8a-e14c-742b-5c6c-018c01ed897e@intel.com> <957ea888-a96b-89cc-29e2-973bb9e36f40@intel.com> <6cec6f68-248e-63b4-5615-9e0f3f819a0a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6cec6f68-248e-63b4-5615-9e0f3f819a0a@redhat.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5927C8001B X-Stat-Signature: sqpbq6dhozkt8qz8nz1sujjuqdwyju1s X-Rspam-User: X-HE-Tag: 1688300803-654518 X-HE-Meta: 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 Wb6SUUtM yIO1XTKwlaTDpmtetKgWvLfKWy8zFppqEMiOc7SngS2K7uLOhHfyTYMeHwtXTb1KDQq+ySg4JXQF2MrZHETmG2y9J+A7/wKBSxZv9TNfasgV+NnZ/ygsYJDOU0B1r8CCe9QzQoP0JkSNH3LrW/3hG/uAmqVL3xK0Dz2oXBDPuUUM/NR/VGB46H2zzXJMkbHxtYAoPHHgWhXm/xCw= 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 Sun, Jul 02, 2023 at 01:45:48PM +0200, David Hildenbrand wrote: > 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. We have a bi-weekly meeting to discuss all these things; it's the same time/day as the MM meeting, but the other weeks in-between. Last one, we discussed the idea of having a 64-bit mapcount stored in a tail page, but having mapcount only contribute 1 to refcount instead of refcount being incremented for every mapcount. We do this trick with mm_users and mm_count (for different reasons, but it's not unprecedented). eg we could do this as: page_add_anon_rmap: if (folio_test_large(folio)) first = atomic64_inc_and_test(&folio->_mapcount64) else first = atomic_inc_and_test(&page->_mapcount); if (!first) folio_put(folio); which is substantially simpler than what's there now. The accounting needs a bit of extra work.