From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) (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 B7A1048BD29 for ; Tue, 9 Jun 2026 17:27:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781026056; cv=none; b=Z016aA1NxoXa6lEEMkD+2Ll1MpvIfqyBKLEdIMrriu4qopGMIniYnJnK1djC0DvgWQcAquLC2vB8TJi6A3EvXIMq1RNklyHQ4s6Jz34OGZ5tTsat4T89vulrNI3LjqfH0erlmNzB8elvN4dOL6dSysMsSgNGDpogOLrF1mlKiqI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781026056; c=relaxed/simple; bh=APKrTKPNr80s6CSlWU9SlzbUSbKP6+ZuKfHgrfiVlA4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=ISoz7ErIQXx5FsVE39gnZd7UG5tbM9nEK0GcC4Hz5QiGBBpy4PeeWp6fGHPIZbzyQ1stXN/OvNG+5nbb1za3vn5afbS+FCM2agYBwuY+qglMw0hFjDfJWyo9XgP00lhAoAyx6E2MPQi/vZy5rrrBbbLd10hNBwTH0ZrU4CAZRPU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=JRbBnebo; arc=none smtp.client-ip=91.218.175.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="JRbBnebo" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781026051; h=from:from: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:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CRCzUiE2hwh6Hqbc2UBSoVPwWk3hneuD42gLXpyxaT8=; b=JRbBnebo77gJlbzFks5VoRzUf9dv8lQdMhZldAuKM9IeGTyIHsU+fGVCsMdWb0ezail2gx OlpKSgQSLB3yCuYdoHXI1CuMYIlCd0txR7DJHJ2+LcSPg09SIx4CruDT8mx8QYs0X/YBw6 vZXg0NHk3qDJICYS6/5iKDCEdtPcak0= From: Usama Arif To: Zi Yan Cc: Usama Arif , xu.xin16@zte.com.cn, npache@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, usamaarif642@gmail.com, yuzhao@google.com, aarcange@redhat.com, akpm@linux-foundation.org, david@kernel.org, chengming.zhou@linux.dev, ljs@kernel.org, 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 Subject: Re: [PATCH mm-unstable v1 2/3] mm/migrate.c: Prevent folio splitting from interacting with KSM Date: Tue, 9 Jun 2026 10:27:15 -0700 Message-ID: <20260609172719.803537-1-usama.arif@linux.dev> In-Reply-To: <18102ABA-0D8E-4DA0-AC30-544E34E445D5@nvidia.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Tue, 09 Jun 2026 10:07:20 -0400 Zi Yan wrote: > On 9 Jun 2026, at 9:47, xu.xin16@zte.com.cn 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. > > > > Understood. You're saying that the additional processing action in split_huge_page > > (remap unused subpages to shared zeropage) increases the scanning cost of ksmd. > > > > However, I still wouldn't simply classify this as a performance regression, > > because commit b1f202060afe increases memory savings through this action — so > > it saves memory at the cost of additional CPU overhead. > > > > If you want to address the increased overhead on ksmd, I think we could add a > > check for the shared zeropage in cmp_and_merge_page, and skip merging when a > > shared zeropage is detected. > > > >>> > >>> 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. > >> > > > > Actually, I have long thought about submitting a patch: add a new interface > > 'skip_huge_page' under KSM's sysfs, allowing users to choose not to split huge pages. > > Just think out loud. Or just skip huge pages all the time unless memory pressure > is present. Basically treat KSM as a way of reducing memory pressure by merging > pages. I do agree with this. I questioned about if it even makes sense to split THP with KSM in RFC (https://lore.kernel.org/all/20260510114001.600681-1-usama.arif@linux.dev/) > > Best Regards, > Yan, Zi >