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Wed, 02 Sep 2026 09:23:26 -0700 (PDT) Date: Wed, 2 Sep 2026 12:23:23 -0400 From: Johannes Weiner To: Nimrod Oren Cc: 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 , Michal Hocko , 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: <20260902162323.GO3004@cmpxchg.org> References: <20260901190123.3511535-1-noren@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260901190123.3511535-1-noren@nvidia.com> X-Stat-Signature: 1pkg3qaa7bje1d7yi81sxxxycjj7eo1c X-Rspamd-Queue-Id: A4060C000F X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1788366209-938196 X-HE-Meta: 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 TmXearzV 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. Nacked-by: Johannes Weiner