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 D00743A9616 for ; Tue, 9 Jun 2026 12:59:41 +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=1781009982; cv=none; b=LliTFU+5OyReEzQCt7/DLHqjIFrPV24EY9fDUPKT94i9TCi+rBJfMSo5NStJBOJ5RuFkTyi7SXhemaacjsO7EvXyKrU+NGMAegwjpfzM3V/ae2lhT7oVTS9E085CG3s6JFwkrc+AkbewkQPXQwwweuBr7G8jIzp1CS3hg6IrdDs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781009982; c=relaxed/simple; bh=yY9HSR4VFe7R1y/hJjsQMZdLAWLol42DOS3iVu1nWIE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IN37drfx6/PTwOi8KQ/eVNCmGVVSbjEwInGo2TcTtGO+hexnp8vNQ+xrN7wVlDaYBNoK2hj6lP+7Lh8a2X3HwHpWmGmZ51ara3JrBobzz+u5kYZsvYzPcFr/r+X0eLmPHQj2OHacR+KwIF+8dsaKZa/dvGhroB/l8hINMbmIQzE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=J+U2e8kT; 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="J+U2e8kT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 87C901F00893; Tue, 9 Jun 2026 12:59:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781009981; bh=4w956L4EhR0QlZHP3ZI2+HoYF1hO6iuotkeOW/qWa8o=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=J+U2e8kT9BYiBG+FapB4TOMQjAdnkPeYBWJ8i9g/qghLw1NgV/iMSxiPAZ0WuVLhZ fial1QCXeMyNJ1DwplSFrrdCqAVEeoNMz9SAknqE9czp4JRMQZlG7gPBicTakhXyYQ Obx5yaMgbp2NuBoZoJm8YGxnP2QaV4XEFCl6cZtRyq+fWvDqaVBzMofAGGne7zrHkJ jO5duelZY/kLy9X32rFOWZiLdi6F11K58SCvQuxDkcqTYkQpF2vH17LYCxoc3hBa6N Tquq+wRPRXtbIr6xzfFuvDdvmUPW5g9+VKgjpXYyU4SKo6HLX2NXdpgvVZZfZmmFLW BtUqGbdSQOKNw== Message-ID: Date: Tue, 9 Jun 2026 14:59:32 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v1 2/3] mm/migrate.c: Prevent folio splitting from interacting with KSM To: Nico Pache , xu.xin16@zte.com.cn Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, usamaarif642@gmail.com, yuzhao@google.com, aarcange@redhat.com, akpm@linux-foundation.org, chengming.zhou@linux.dev, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com References: <20260609114619.144416-3-npache@redhat.com> <20260609201202615oT6yd_LtIoKkGLksFaCha@zte.com.cn> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 6/9/26 14:57, Nico Pache wrote: > On Tue, Jun 9, 2026 at 6:12 AM wrote: >> >>> Since commit b1f202060afe ("mm: remap unused subpages to shared zeropage >>> when splitting isolated thp"), splitting an anonymous THP remaps all >>> zero-filled subpages to the shared zeropage via TTU_USE_SHARED_ZEROPAGE. >>> This flag is set unconditionally for every anonymous folio split, >>> including splits triggered by KSM. >>> >>> When KSM is enabled with THP=always, this causes two regressions: >>> >>> 1. use_zero_pages=1: KSM calls try_to_merge_one_page() which triggers >>> split_huge_page(). The split remaps all 512 zero-filled subpages to >>> the shared zeropage at once, freeing the entire 2MB THP when KSM only >>> intended to process a single 4KB page. This bypasses KSM's >>> pages_to_scan rate limiting, causing ~1GB to be freed almost >>> instantly. >>> >> >> Why do you see it as regressions? > > Since the zero-page remapping was introduced our test has shown the > following behavior changes: > > With use_zero_pages=0, the merge rate drops from 60MB/s to ~6 MB/s > even after raising pages_to_scan. The KSM merging is now much slower > and CPU utilization has increased. > > With use_zero_pages=1, ~1 GB is freed almost instantly, and it no > longer respects the pages_to_scan behavior. > > Even with just this patch (1 & 2) or the RFC linked in the cover > letter, the issue no longer occurs. > >> >> AFAIU, KSM and THP do often conflict with each other. THP tries hard to collapse >> a huge page (which may contain many zero pages). If KSM is enabled and part of >> that huge page is mergeable, it can easily be split by KSM, rendering THP's >> efforts futile. >> >> Therefore, in our actual production environment, we typically avoid making the >> same region both KSM mergeable and THP always. > > THP=always is a global setting used in many production environments, > so these features now interact very poorly together. Red Hat documents, though, that both things in combination are shaky: "As KSM can reduce the occurrence of transparent huge pages, you may want to disable it before enabling THP." [1] (for RHEL 6, but nothing should have changed in that regard) But yeah, having KSM and THP enabled at the same time is not uncommon. [1] https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/6/html/virtualization_tuning_and_optimization_guide/sect-virtualization_tuning_optimization_guide-memory-huge_pages -- Cheers, David