From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 408193C7E13 for ; Mon, 31 Aug 2026 08:47:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788166059; cv=none; b=NqZxnaS3MkO+hdQbPeqn4Pkzi1dhFodBpeSJ1HT2zVUTRfhp4eaQI0hp1EimfHGLyoC7bjF0IfVEiCZ3f2ytNR/E9VnxfnyoCvTvtnQAdDUTrkfR6Qqhq8w0JdNgGvRJxs7r9Cp0mGu5MAJXkJ0TmUSOkgJndXQsJ5MuVKtrE00= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788166059; c=relaxed/simple; bh=wbu8RT5VatC4TtVuOFTjBePWXgugb9wnJsoIyMAKsTY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=k+0OllDetsHa2Y/toHun/AJJdL8fDg050jDoJEv4hbD5LRis8KGvdzGL+ykC7oV3tFKFUGVYiSP9HHMLyQNOHE7VT6xWIuIKIpFxKs1xAX608fkyyl83qx5lTY3+izJqFPKKvyb/gnMTzXLQqJH9/Jbf8CNKD+7GYM+AdS5nDsk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NXbrSDFa; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NXbrSDFa" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B42C1F00A3D; Mon, 31 Aug 2026 08:47:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788166058; bh=hgZaDb3jHysBqG5gB9RY2SL7OLUO+d0tIyfCa7KbGt8=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NXbrSDFa1/9U0XSEHM40KgGB4WfANiAmF0LQTXjTDdWlWlPhLkWZ3ugagGcRAto5+ rXJKReHCLz63eCdtPj2hr7eYaNLCUdTKkLmsMUlAZcB5/IwDITZguvkWGOk1FqyiIB iQGyapiQAaXt6aZbeUm5HRN8ERGukAUf/tDFYFdjeW17+K7HM/qTUtpZV3tT4hXjP8 QMCyWe2QiD53RtiUa8EwUxrEF1YB5+L8PH87VAIuXqM9tqy60PNfpQENKqX2jqVGpa TqwzSqyOupIayHdxqUchNOZSWnWwX0YDKzFEH3s7PQU7fiYq1Ur97E0Vf2VqzAyDCR 5TjZBvs9QtjGw== Received: from phl-compute-05.internal (phl-compute-05.internal [10.202.2.45]) by mailfauth.ams.internal (Postfix) with ESMTP id 41DE31980082; Mon, 31 Aug 2026 04:47:31 -0400 (EDT) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-05.internal (MEProxy); Mon, 31 Aug 2026 04:47:35 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTF8AnKVRZxti1PyxKUQhgoh6fkIiLRKSDqX5+0I8QAvrjxkIUL+BQ10yr9hmtk99G pMpYdorUKGcR+yTaZtXLylI2S6vtx4c+chOuEgbSUZjQlPnmrZ9zCj1rF0M/Yhb12AbdNc BYpMQpkYUl3fVZYnEfpOD6aa73TS5tCd/nmbLMHGX4lCdQM9rLl8kA1v4wdRRCrjW+Iv8U MpAoosOpq/o8hsUUe67K3OOrl40WcoRYYwHbeVrayNXXWVzchIqBNGgr+6ISGiUcQ13m5N YaCSSLjEFub8qC9Ka1FGvDTLqHGfpp3aOXUce5CGJR8pTEpEb7ULvXQfHaLl4HgPD3AzaX oSUBJQbvX3eoVQE0dM0CWzCnBnKphJo3yTYs0NvARQoAtsNkrIfngil/s9r6SWIoIMS0mj 1RmAkc7+E91fRKttvamj8f22Squa84WukcOR2snrRLHOnStRSjUvXCoU0g8yP/+/VDfwG2 cJDXNllLwDLe139QxvJoh2nYWhloaffjY1l49Yb2dRGJf2TjxTnoV7bic0HJb6plKyg3xA YU596B3AAriWI39a81wNE5JoieM11LKwX5C7go3LBsdrOqopTFiNYXWQF/EZ4DBMXzUt05 7ppsHkzY2s8/dR5kLmlKkletfSlW0IjXb5rD09wr//SvMSJ0wPohPv6gIpMA X-ME-Proxy: Feedback-ID: i10464835:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Mon, 31 Aug 2026 04:47:30 -0400 (EDT) Date: Mon, 31 Aug 2026 09:47:29 +0100 From: Kiryl Shutsemau To: Nimrod Oren Cc: Andrew Morton , David Hildenbrand , Lorenzo Stoakes , 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> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260831075635.2244437-1-noren@nvidia.com> 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. -- Kiryl Shutsemau / Kirill A. Shutemov