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[109.81.17.247]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48436646548sm10478328f8f.37.2026.08.31.02.20.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 31 Aug 2026 02:20:31 -0700 (PDT) Date: Mon, 31 Aug 2026 11:20:30 +0200 From: Michal Hocko To: Nimrod Oren Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Jonathan Corbet , Vlastimil Babka , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Shuah Khan , Randy Dunlap , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Kiryl Shutsemau , Nirmoy Das , Dragos Tatulea , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/khugepaged: cap min_free_kbytes recommendation at 1 GiB Message-ID: References: <20260831075635.2244437-1-noren@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260831075635.2244437-1-noren@nvidia.com> X-Stat-Signature: h9hpr3p1ic74cybcx74cdjuxgkn4jsjd X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 82D86C000A X-Rspam-User: X-HE-Tag: 1788168033-232411 X-HE-Meta: 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 Wy5W6Pkc bycnjzPsIjZVhdoj1ErbIKQtVgy8DfecxpmlrMg2ymQAv6RbVklbi9hTK3NkFmYsSOOzj0RTAObbTh2sR33rucju9rpP6h8YVrqRHA53r5VmkC2jpQUXfm9S2vb1iI6TOJyl37vN55NJmXyJJnHfveojjIhELs2opG6U5mMigM8TUBMwfBO2CHoAqvuXLw9nGRCF/UlnUBXbc2Ipbi4EiBJ+PBrUmGQI1ut6wOuyC08KS8ShmpO3itcagwek5qv/XGJsgJrunsxDhw4RbS+HVqS4jKkwe3+djZ+JVEyV9u1ewv++xBS8xHCrpWo5n+LEO08mhPTIgHWAS82bpelHow+3TlUCYGWAQkjx9ObexpAFRWdWP+t1fEOhOXkCLe3Nmq8oiRNm7qvpFn7hgUGvSyRh6gA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon 31-08-26 10:56:35, 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. > With MIGRATE_PCPTYPES equal to 3, the formula accounts for 11 > pageblocks for every eligible populated zone before capping the result > at 5% of low memory. > > This is reasonable when a pageblock is 2 MiB, as on common 4 KiB page > configurations, but scales poorly with larger base page sizes. With > the default arm64 pageblock sizes, the contribution per eligible zone > before the 5% cap 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 > > Consequently, min_free_kbytes can reach excessive and unwanted levels. > > Add an absolute 1 GiB cap to the recommendation, in addition to the > existing percentage cap. This bounds the automatic recommendation to a > sane value on systems with large pageblocks while preserving existing > behavior for typical systems with 2 MiB pageblocks. I would argue that the whole model of increasing min_free_kbytes for THP is wrong. This will trigger memory reclaim sooner and make the THP availability more likely but this was at times where we didn't have pro-active compaction and many changes in the compaction. So is this actually needed in general resp. only large pageblocks systems? -- Michal Hocko SUSE Labs