From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D0E47C61DE2 for ; Mon, 31 Aug 2026 08:57:18 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id DB3A56B008A; Mon, 31 Aug 2026 04:57:17 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D64806B0095; Mon, 31 Aug 2026 04:57:17 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C7A2E6B0098; Mon, 31 Aug 2026 04:57:17 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id A51C46B008A for ; Mon, 31 Aug 2026 04:57:17 -0400 (EDT) Received: from smtpin07.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 13327C03E1 for ; Mon, 31 Aug 2026 08:57:17 +0000 (UTC) X-FDA: 85160960514.07.205524E Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf23.hostedemail.com (Postfix) with ESMTP id 577FD140008 for ; Mon, 31 Aug 2026 08:57:15 +0000 (UTC) Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="EyqF/SjR"; spf=pass (imf23.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1788166635; b=Cm5BIktHiNMPZUPVDV9Cq4gh6Ffl183bDwpU+BJrO1s7syb65fC5Oa1cZfkSDee9Df97Xi 07MSV1XDY7amhASbhI5VvOAR1pUr1HE2Z65V6q8uWIMA2PyLfuTLDp32LPuaZVuRCvSSu5 W32TarCweMMwbkGwfKgcAzdzULFNZhc= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1788166635; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=YpqPq4bvtMsOVeG0UN+yy/NuoFZ+H+t/+49d1u/I1io=; b=K4Ew5ET+PauBAzjua95zBgXvc52ydFIvIzVUYw49Z4ZNVY38om1M4beXNisoNuRqA8g+j6 yn8K6yYd21raiisJoZ9pEO3LfN3F+GwCrSy3dHmM9AeuaZ6NjsjAJBV/4n4JvA8DrkeLkx OO9bRG5Q21p5HFD5CHEtBjrXUVZ1DAQ= ARC-Authentication-Results: i=1; imf23.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="EyqF/SjR"; spf=pass (imf23.hostedemail.com: domain of ljs@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id E0DFB411B0; Mon, 31 Aug 2026 08:57:13 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1274C1F000E9; Mon, 31 Aug 2026 08:57:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788166633; bh=YpqPq4bvtMsOVeG0UN+yy/NuoFZ+H+t/+49d1u/I1io=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=EyqF/SjRvWc/h8CQwUE1TihCdfdXAUbBtv3y4A0dT6UbuMtRzhhG0ezD1YP8W6IB1 D1UC6+/yv2T3QyDTv/QKSDkuhr16a7cryNgwZ5B6fcR9kxnoN3Rzu1psKdUKxDkIYd 54XgUOPP0PK3+aGg3J5NrGYHZ2Olr6u5urt8HbFyNnKED5mKtDkI/30RyVIOpHeDT/ 5ZCfkLhsWRwLFq0zE0aJsZM/2e1qpUcEmHgFIPds4GXPE1n0n8Q4v4jVTv8c8x70qs 5iGeZvi71UgCbxIg4rPs/lKJWa/iDnwILmdscXOHeUODYzlfByh+0KRz1cCHK442nz QvYs+X0zqjPQg== Date: Mon, 31 Aug 2026 09:57:04 +0100 From: "Lorenzo Stoakes (ARM)" To: Kiryl Shutsemau Cc: Nimrod Oren , Andrew Morton , David Hildenbrand , Jonathan Corbet , Vlastimil Babka , "Liam R. Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , Randy Dunlap , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , 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: X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 577FD140008 X-Stat-Signature: 55s66up19u8m3p67dycf859jz6ogiha7 X-HE-Tag: 1788166635-546795 X-HE-Meta: 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 a/ehc7wS bbz/vWchwhMxu5uhGDbNULkNRj233h1pEwvOIqBl/hQvvhrr1EvWa+FtyoFha0R1sDdblbz3SbrLY8zjJ35TEW8ehcBNAli3+3Yn6sheiMyJ3en0zTmd+07BCyX7Jae4q5TeNZE7efd1UmSmpvnKgmKSjSuTK/1xVdeGv1Tk7GBgc/w8ZLrEZ4v9bRTuWc/oDBjZtufITRkLl2C7S/yMjrEIV0M+lP+lbdDF4TEy9SxzeLEBSWuFEfLxXtddNGoNuO7GapjqF7V/MGWj6NC+H3Ky2hrf8IMNUVjbXJb0HKUhnPZo= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Aug 31, 2026 at 09:47:29AM +0100, Kiryl Shutsemau wrote: > On Mon, Aug 31, 2026 at 10:56:35AM +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. > > 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 am not against an upper limit for min_free_kbytes, but I think the > root cause of the problem you see is the size of the page block. > > We at Meta switched to 2M page blocks on 64k ARMs recently, targeting 2M > mTHPs. A 512M pageblock is not practical. I mean we do keep looping around on these things as 64 KiB page size is fundamentally a big problem for the pageblock model :) I don't love the idea of just making page blocks arbitrary smaller for higher page table size, because hey the intent is to get a PMD-sized thing. So really I think to do it 'properly' you'd need to rework how that whole thing functions. For me the approach here is a practical trade-off - if the intent is to cap at a sensible value, then just cap at a sensible value. > > -- > Kiryl Shutsemau / Kirill A. Shutemov -- Cheers, Lorenzo