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[109.81.91.122]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49cee5d6029sm71680715e9.6.2026.09.03.05.52.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Sep 2026 05:52:50 -0700 (PDT) Date: Thu, 3 Sep 2026 14:52:49 +0200 From: Michal Hocko To: Johannes Weiner Cc: Nimrod Oren , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Zi Yan , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Brendan Jackman , Hugh Dickins , Nirmoy Das , Dragos Tatulea , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] mm: remove min_free_kbytes adjustment for THP Message-ID: References: <20260901190123.3511535-1-noren@nvidia.com> <20260902162323.GO3004@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260902162323.GO3004@cmpxchg.org> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 1C3B240008 X-Stat-Signature: 6qoyweem9otnz1zyb174ao9ua3pbjdwy X-Rspam-User: X-HE-Tag: 1788439971-27098 X-HE-Meta: U2FsdGVkX18OODrHz3T82d/v22fXAPpchHQnZC7uAINg0L+iY/cNO4hbS7f8bSxR5Sj6fULtri9aSI8O3OETXxWNR61/EFTZouxFoC60JOW4GUuUhkiQl+YYXBoPvV8V9qlcJu7qkt3+aSViBqqFL4lGgwm0C0x+3qQsuzqTrJOeg3ZWLMzo2k2ySFfbExxJbUoFxk3CBONRPGurfJpUcniOUXu5NjTw7btkC2rdCJme3xqmonkknYUbrbr8JxyKIABqlMYFdrxInjGKab3xrqytzVcF81AegVN0ziIKIaF+3F3FKyq0nmGtZerwOo+VAUzVQy/xG9gHq9wIQJtG2239LhlQrqTKJS4FBTzqeMdwxNVug1/CcO8sjGyqJ/+6kVbe2i5gCzRQ8Ux1mJlE0MLlXVAurhzAkHW5NuVi22M/w21rO6i+8sA0oZ40IP3GSuV8lFxsEa0HmH2961BOGtj7kcBlnDTYBdNL1WENrqehYgb1CyoMsqvOQ5cDv4keEmkdgp3iBu3sUEwwuM6Eg0Bp6/NiKB9/lzFa87MAG1sVbZiJBLQYrWEfzMwymVbkQWkt5VyjCi+Z9dBLjZNDpqHlKCWcJRExZaLpv0fJV456IZQVDn/rwKtC6RE+oXc8d7d6/h1oVnCRxR4QxcPTRRuqjFeio0rXKWcAlsiKipqXnSs0q2l1pfIhSnT+2Kdt8vRSa7G4mp51358ClCdwEuRNfY58bNMcDfcKSJXGkkL4ZRUFz125B7QYjAcmuUOyGR7cPdCIpapqJKmDfdrylAfxmt4u/h9NnO2Ir/S7cIJquX4zN5Fzm1r90XauTyLE6m1zYkLK5/KqKuwV46Vycjx+pdtkiDvl2MMGXxUzqVLYq/Pp/iPoXA+Ilm0lYwdLxxC8mB+m2lb2rcKLzhEnK+FyZxcLW7ox1aa2rtsF5Ewn+jc1S6WGMjipwLaqJBPMQYeI1oKbL6BAVSpuO2o 6tuZU8LR v2xG24BU95Qo+FgmghdscQ2TK7A/mmWUC65AvN1Ryf/edEsuhFmKJByQX9Xa5LTxU/PTfT0lUy+L9NOGPu3CnHUfNEtYuYonCVDMHdJO3ieI9WB5MVcxWeHdyWjCKgLThb//Pm1djw9M6+nsZhkJc56C415mtwN62Gp9ADMiFbjl+xio2qbnCvzRSScA5SHVBJPwjtHMQqpXN+DKwQpLCPPOFgB+TtqN3Ia5X2XRliO8QU/IugaFaeXBRfm/gRqrlvFA6OTT0pHzy8eUYjWwEbSie9QJSQtg2tsE4mg+xTlyWNu3lkn0BkHeO86s6FJPkvxk1HCEsTDvOIEH3RUcypAbUftGcbUih4xxF1GwbjKHD8vG81HXCDzWpfHQzZxZSxaQhEk9pO1nMlsK1KwqI6ELXvo8mDBbFu7XONb32/uIaQ0Nniax8UJk6Rgsuv8a13uG6lwdCAVNF0RGE8Q4FV5RlOCmGji0Aqj/7sAiwTVoEw4/Ag0eLGpP5oOl2kb+hYtomYGxFXwYebHxo0JT2DeNg3W/rCTIIUVWa1R8ejaWikuhOBTJPjislWBhqV32b4/ng Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed 02-09-26 12:23:23, Johannes Weiner wrote: > On Tue, Sep 01, 2026 at 10:01:23PM +0300, Nimrod Oren wrote: > > When THP is enabled, set_recommended_min_free_kbytes() may raise > > min_free_kbytes using a heuristic that scales with pageblock_nr_pages. > > Commit f000565adb77 ("thp: set recommended min free kbytes") added this > > heuristic to help keep pageblocks free and reduce fragmentation for THP > > allocations. > > > > The recommendation scales poorly with larger base page sizes. With the > > default arm64 pageblock sizes, the contribution per eligible zone > > before applying the existing cap of 5% of low memory is: > > > > 4 KiB pages: 2 MiB pageblock, 22 MiB per zone > > 16 KiB pages: 32 MiB pageblock, 352 MiB per zone > > 64 KiB pages: 512 MiB pageblock, 5.5 GiB per zone > > > > Even with that cap, min_free_kbytes can reach excessive levels. > > > > The automatic min_free_kbytes increase predates proactive compaction > > and many subsequent changes to compaction. Given those changes, > > increasing min_free_kbytes for THP by default is no longer clearly > > justified. > > > > Remove set_recommended_min_free_kbytes() and all associated > > recalculation paths. With this policy gone, min_free_kbytes is > > controlled only by the page allocator's default calculation and the > > vm.min_free_kbytes sysctl. Users who want additional headroom may set a > > higher value via that sysctl. > > > > Link: https://lore.kernel.org/r/20260831075635.2244437-1-noren@nvidia.com/ > > Suggested-by: Michal Hocko > > Signed-off-by: Nimrod Oren > > Just to summarize my take from the subthread with Zi: the premise of > this patch is to roll the regression dice on every THP setup out there > because certain ARM configurations result in a questionable pageblock size. > > I'm not against carefully evaluating and testing out today's need for > set_recommended_min_free_kbytes() in real world examples. But this is > not that. I do hear your concerns Johannes. They are valid but I am not really sure wht kind of testing would prove such a change to be safe. > Nacked-by: Johannes Weiner -- Michal Hocko SUSE Labs