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Mon, 25 May 2026 10:29:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id xm2QMH4kFGpRRgAAD6G6ig (envelope-from ); Mon, 25 May 2026 10:29:18 +0000 Date: Mon, 25 May 2026 11:29:17 +0100 From: Pedro Falcato To: Chengfeng Lin <23020251154299@stu.xmu.edu.cn> Cc: "David Hildenbrand (Arm)" , Andrew Morton , linux-mm@kvack.org, "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Chris Li , Kairui Song , linux-kernel@vger.kernel.org, regressions@lists.linux.dev, ljs@kernel.org Subject: Re: [REGRESSION] mm/mprotect: shared dirty PTE toggle takes ~1.6x longer on v6.19 than v6.12 Message-ID: References: <662955ba.f499.19e3b2cf478.Coremail.23020251154299@stu.xmu.edu.cn> <7bab1d67.f5c0.19e3c0979ea.Coremail.23020251154299@stu.xmu.edu.cn> <4fbfa9d.10a05.19e4eed247a.Coremail.23020251154299@stu.xmu.edu.cn> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4fbfa9d.10a05.19e4eed247a.Coremail.23020251154299@stu.xmu.edu.cn> X-Spam-Score: -4.51 X-Spam-Level: X-Rspamd-Queue-Id: E907D6712F X-Rspamd-Action: no action X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; ARC_NA(0.00)[]; MISSING_XM_UA(0.00)[]; RCPT_COUNT_TWELVE(0.00)[16]; MIME_TRACE(0.00)[0:+]; TO_DN_SOME(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo]; internal_greylist_whitelist(0.00)[2a07:de40:b281:104:10:150:64:97]; DKIM_TRACE(0.00)[suse.de:+] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Flag: NO On Fri, May 22, 2026 at 05:03:44PM +0800, Chengfeng Lin wrote: > Hi David, > > Thanks for the pointer. I tested the current akpm/mm mm-unstable branch at > 444fc9435e57, which contains Pedro's v3 two-patch mprotect series: the > softleaf refactor and the relevant small-folio / nr_ptes == 1 changes. > > I first ran a local sanity check, and then reran the same shared-dirty > full-range toggle workload on the lab machine: > > kernels: v6.12.77, v6.19.9, akpm/mm mm-unstable 444fc9435e57 > QEMU: direct boot > lab guest CPUs: QEMU_SMP=1/2/4/8/16 > lab guest memory: 14336 MiB for 1/2/4 CPU, 16384 MiB for 8 CPU, > 32768 MiB for 16 CPU > repetitions: 9 > order: interleaved > coverage: disabled > > The primary metric is cycle_ns_per_page, lower is better. Here "cycle" means > one workload iteration, not CPU cycles: > > CPU v6.12.77 v6.19.9 mm-unstable mm-unstable vs v6.19 gap closed > 1 336.1 532.0 497.0 6.6% faster 17.9% > 2 369.2 581.9 503.3 13.5% faster 36.9% > 4 355.7 587.2 524.2 10.7% faster 27.2% > 8 369.7 583.6 534.2 8.5% faster 23.1% > 16 374.8 607.1 547.8 9.8% faster 25.5% > > The 1/2/4/8 CPU rows completed 9/9 runs for all three kernels. In the > 16 CPU row, v6.12.77 had one QEMU failure, so I would treat that row only > as a supporting trend. > > So yes, Pedro's small-folio work does reduce this synthetic shared-dirty > signal in my setup. It does not seem to remove most of the gap to v6.12.77: > looking at cycle_ns_per_page, it closes roughly 18-37% of the v6.12 -> > v6.19 gap in the clean 1/2/4/8 CPU lab rows. > > I also ran a separate state-shape audit, because the MADV_PAGEOUT follow-up > showed that a timing delta can be misleading if the compared kernels are not > actually operating on the same page state. For this mprotect workload, the > successful runs across v6.12.77, v6.19.9, and mm-unstable all used the same > 4 KiB shared-dirty PTE mapping shape: > > expected_match_ratio = 100 > unexpected_results = 0 > final_vmas_avg = 1 > present pages before/after protect = 16384 / 16384 > AnonHugePages = 0 > KernelPageSize/MMUPageSize = 4 KiB / 4 KiB > THPeligible = 0 > > The state audit used the same 1/2/4/8/16 CPU and memory matrix, with 5 runs > per kernel. The 1/2/4/8 CPU rows completed 5/5 for all three kernels; the > 16 CPU row had one v6.19.9 QEMU failure, but the successful v6.19.9 runs had > the same state-shape values. > > I put the follow-up summaries here: > > https://github.com/lcf0399/linux-mm-regression-evidence-2026-05/tree/0c0e2d9/mprotect-shared-dirty-toggle/mm-unstable-lab-sanity > > https://github.com/lcf0399/linux-mm-regression-evidence-2026-05/tree/0c0e2d9/mprotect-shared-dirty-toggle/state-audit-lab > > Given Lorenzo's question and the synthetic nature of this workload, I will > avoid treating this as a strong regression claim unless I can provide a > standalone reproducer and/or a narrower bisect. If this remaining signal is > still useful to characterize, I can prepare a smaller standalone reproducer > or try to bisect the remaining gap. Yes, if you could give me more pointers (and a simpler repro) I would be happy to take a quick look. Otherwise there's not much I can do here :) -- Pedro