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[95.117.105.1]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e25835b1dsm3374756f8f.40.2026.10.20.22.56.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 22:56:28 -0700 (PDT) Date: Wed, 21 Oct 2026 07:55:55 +0200 From: Karl Mehltretter To: Sebastian Andrzej Siewior Cc: "Vlastimil Babka (SUSE)" , Andrew Morton , Alexei Starovoitov , Alexander Duyck , David Hildenbrand , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Shakeel Butt , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/page_alloc: skip shuffling and reporting for no-lock frees Message-ID: References: <20261005063515.6312-1-kmehltretter@gmail.com> <20261020110948.wv_yGAhQ@linutronix.de> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261020110948.wv_yGAhQ@linutronix.de> X-Rspamd-Queue-Id: 09638180004 X-Stat-Signature: keum8xwtnezym35f1bndxsmyoj7gcctn X-Spam-Status: No, score=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=no autolearn_force=no version=4.0.1 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1792562190-558234 X-HE-Meta: U2FsdGVkX18JboR78L/QbOkHyAbXoSOo2ZWa/pDokv2yK0OA+du/ep0pLPFEwjBv+jb+3dXiUjM/r8oz1xlv6z++uCwApEbIY3/e6CzqLI/JIpyekjddfE+lSJqodohsquWfYCUmSX8rBiI2Vj2YI8xVbFzEVkbgt9CWG+2l8mrgfna8bSU1ploK2bXlYOvvvGHOQvDbPnH960g6/OoS2hHnSuXUnYukd2Ii1IWU1O/I/M4IBDdJS1BOaVEh+Z8zU9VBJWXTph5xJtd+kmcI8SABy14NtoJX9pzXLPkHqgsMQ7KAQT+QfR4KLkTs8ttOAsQmUA//9hmLZ0ytfuxJzBamQl9FfQZ4bhCu+6LYAjknIVU5inRhOra5/C4/vYR03neLfhtLWUMpUDq8VbEnbpIOuJp6aTBBJAdTOGpUP3JEscbw19XIugf/nG0mtqXdQL8oXFYo5WBkT3gDKFqLPIJSRD2f3GmFmFt+fpdYbDtrmn9ySItMUQvukgN/LV1QOS/wbdZBO0Tw5y7WUE5lL9ULKQ7TeD2rT4VvLQ1x+rS35A9of4Lqq0tboLkzSmJ1v58cHED6/WrLidJ5BSkR+CWlBUD/0qllxAFi9THa41Zu4NzIUVL6afYqucuR4jC/lNagI0IBVksrjartl1V96cJIg+D8GotxBxL+ODppKmPWb1HzoORNtFVpzARI/T8t/8psogN4vay3oZ3ngDWktQz974GzuzOZXfydIlSVS4G8SYC5IpgEwT8kDz0yQSs2VtX2pCUTntj1qOBGy0KBmvUjq6mIFW1neFcnZ9veonI2fo0oAbUvUlvq2vRgPyx4PLyxs3En/fqYGu2yb0opQQ68TL8V6UbcBLwQSFAKbaMYhr9s/EIjsDyEX4WpszFzx7CZNxBCLCXo97hYA5uzZsUz7Y27Rsdns+ZxGzqDOerX4tHGpzDkp5zpFIOKp81mcG84/BqlYtt3lHIF77u 6YzFEr9i b5wpyg62InERNsCAOri8OnAAtu9KQjR0JYa5Fv5muuE7ZTWSpDhzOELa9TDSUoYVaVB1xJ1jmlWgpC8Wd3uXChNv8Eji/Xf1rkxmFw7O4mEhrgdsOshtD5cPHBatf+xry8BJUZoLtJGWNdba3vERHgdxle8qozQnEDvc1z1RuEQpS2k/r2/l80TnKNF+Ve/8X3P1n+JC1DL5k5zKLFec4psv8wX1LuqJMcmJVS4j4H3nQyVhi0mQKM+CxyLyFzBj0It3FZ8RG+x6mBqTxPtWQoGuBpdy2GWhqiU9RPJluPiFFS7PTrArqlyq0eNmelc2R70V+X/vWCCvixtZ8MliovDvAZ05n1awbP0yLaXEllZ69SLH2yCYr1fLy40MXBDumq9KL6w1rK6hchaXxxZEh2OBEnr1PcsxNxnwD6nOjGvhsY/B1EODc3ElRUl0/+Ia9l5WiTivU86Q8tuwHIt5tJolwRakUO5Y/FxNa/59WNQsK+e0= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Oct 20, 2026 at 01:09:48PM +0100, Sebastian Andrzej Siewior wrote: > On 2026-10-20 12:02:01 [+0200], Vlastimil Babka (SUSE) wrote: > > > An instrumented PREEMPT_RT lockdep run forced a random-bit cache refill > > > while calling free_pages_nolock() under a raw spinlock. The unpatched > > > allocator reported: > > > > > > BUG: Invalid wait context > > > > > > The attempted lock was batched_entropy_u64.lock in get_random_u64(). > > > Lockdep showed the raw test lock and zone->lock both held. > > > > In slab we solved this by using prandom - commit a1e244a9f177 ("mm/slab: use > > prandom if !allow_spin") > > Could be better? > > I hated it at first but now that I looked longer at it, it is not that > bad. > For v2 I can pass allow_spin to shuffle_pick_tail(), keep the cached get_random_u64() path for ordinary frees and use per-CPU prandom for FPI_NOLOCK. For page reporting, the issue is NMI rather than raw spinlocks in general. free_pages_nolock() may be called from NMI, while the reporting path may queue delayed work. I would keep the FPI_NOLOCK check and add a comment explaining it. I will also put is_shuffle_order() first as suggested. Karl