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Howlett" , Lorenzo Stoakes , Vlastimil Babka , Jann Horn Subject: Re: [PATCH v3 00/20] mm: MM owner tracking for large folios (!hugetlb) + CONFIG_NO_PAGE_MAPCOUNT Message-Id: <20250303144332.4cb51677966b515ee0c89a44@linux-foundation.org> In-Reply-To: <20250303163014.1128035-1-david@redhat.com> References: <20250303163014.1128035-1-david@redhat.com> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 710C840006 X-Stat-Signature: nyw83oeidbdxj3iskradmyaajrqzxgkw X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1741041814-381169 X-HE-Meta: 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 XtWFF6Qg pBQMNhU6XTwIFJfXfXzb1F1CE2Bg/yaP6ObQ4JutY1BrNh9ZV+QEUi8Hbyav5Qu6/rR8+kfrwuCJSU5lvPWNp5EFHpPIsCUms0uzWaTpe/s2F83oJcvM9vDPeJRFFC0k8NL8vVrL2UfIvLssfSpSI5nKT3N5cnOkZBjvK1bloestBnvbSji03WkwlStp+9EQpIbgBvertBWDhSywwrOjM58upruMZMK8IXMPb2EKp8LLwI7aaEp1ybj013VJlY0PIAQ0mGxJCGp1ToiycD+88+VD4kvPsgDGx4ue13gtUnEeG8Fboh6s08Qve/iipSCl86GCiBJ2T+9CJ0jaOotFc4gd/xn8T8ztSlGPJzcyAK9/7Udk= 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 Mon, 3 Mar 2025 17:29:53 +0100 David Hildenbrand wrote: > Some smaller change based on Zi Yan's feedback (thanks!). > > > Let's add an "easy" way to decide -- without false positives, without > page-mapcounts and without page table/rmap scanning -- whether a large > folio is "certainly mapped exclusively" into a single MM, or whether it > "maybe mapped shared" into multiple MMs. > > Use that information to implement Copy-on-Write reuse, to convert > folio_likely_mapped_shared() to folio_maybe_mapped_share(), and to > introduce a kernel config option that let's us not use+maintain > per-page mapcounts in large folios anymore. > > ... > > The goal is to make CONFIG_NO_PAGE_MAPCOUNT the default at some point, > to then slowly make it the only option, as we learn about real-life > impacts and possible ways to mitigate them. I expect that we'll get very little runtime testing this way, and we won't hear about that testing unless there's a failure. Part of me wants to make it default on right now, but that's perhaps a bit mean to linux-next testers. Or perhaps default-off for now and switch to default-y for 6.15-rcX? I suggest this just to push things along more aggressively - we may choose to return to default-off after a few weeks of -rcX.