From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (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 6E0AB2BDC26 for ; Wed, 15 Jul 2026 10:54:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784112893; cv=none; b=AP0K1ZVNj9SLD3Z8NFAQ4S+dXS5LmvdCt6sn+XbReX35vzkFh2xxHMyD/z1vzB3AgcAGFj4f09VYKhkNZ1BVEGW9N2TLUlh5SmtoyCNmT3F4UwJCocYiLWyzp91jDoProRmy/gfOcgHDIG/0/snjM533swjIeRsrwwlE9mm/LZI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784112893; c=relaxed/simple; bh=vLoUC+/wBzHyfUt1EJaNU0bV0muznGjhd3QpjZGK6ns=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=XN23xRH/1QiKJHOfI6G76FFZH0BthGLSVrfMGnwJXs+HwA8/NxFFmYNRNjHwVpygxYBMWGfL2WLO+2O42QI2ugI0bP7pzrA2UKPtt0L5QEicSoP8EzxjW0ZcNxHPb8O5nIcgvs9z66xqXcb2aB5vjVwyzp5SBwhC5HnegHX58zM= 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=jagq1YVU; arc=none smtp.client-ip=91.218.175.183 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="jagq1YVU" 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=1784112889; 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=70Z5NezoUH2RJlJVI3JA/0j2Eg0rD+q2bAJ3zEsLrN4=; b=jagq1YVUsHNqTIL8gPgg0nmxCuxZois9P4j0k7ZR4PfYTYED1sfSvHB3lgfr7ExZl/gLno ZwjS4uegfSMlEUxhd1JHymHJkVuz5B8uB1I8HhOP+8Is8F3ZCeXJ+p8LntlYJbmcwg9YI5 7zwIgenApe1eeKa49so47zudgDoBRdk= From: Usama Arif To: Nico Pache Cc: Usama Arif , Zi Yan , ljs@kernel.org, david@kernel.org, xu.xin16@zte.com.cn, 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, 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: Wed, 15 Jul 2026 03:54:33 -0700 Message-ID: <20260715105440.2946210-1-usama.arif@linux.dev> In-Reply-To: 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, 14 Jul 2026 22:05:31 -0600 Nico Pache wrote: > On Tue, Jun 9, 2026 at 11:27 AM Usama Arif wrote: > > > > 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. > > This would be a good intermediate solution while allowing backward > compatibility. > > > > > > > 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. > > Hmm interesting. We should decide on future changes before committing > to one direction; however, i think the immediate solution would be > what I currently have prepped for v2. > > My v2 follows my RFC alternative approach: skip zeropage remapping if > the split comes from KSM. It also contains the patch (3) from this > version, which skips zeropage remapping if the shrinker is disabled. > > > > > 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/) > > This is indeed a good question but I don't know the right solution. > > Perhaps we can discuss it here. > > So far here are the potential solutions: > a) skip zeropage remapping if the split comes from KSM > b) skip zeropage remapping completely if the shrinker is disabled > c) Disable KSM THP splitting completely > d) add a skip_huge_page sysfs toggle > e) only scan THPs when memory pressure is present > > (a)+(b) would be my V2 which I can send out whenever. > > (c) is the nuclear option, but perhaps KSM for THP is rather > inefficient and pointless I really like and want (c), but I know David has already pointed out that there might be someone already deploying and expecting this. > > (d) could be used with (a) and (b) and allows more flexibility than (c) > > (e) is a more dynamic approach, and could be combined with (b) and (d) > (e) sounds like a good idea, but then KSM effectively becomes a THP shrinker? > Let me know what you think :) > -- Nico > > > > > > > > > > > Best Regards, > > > Yan, Zi > > > > > > >