From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-133.freemail.mail.aliyun.com (out30-133.freemail.mail.aliyun.com [115.124.30.133]) (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 3D3F42820A7 for ; Wed, 25 Jun 2025 10:03:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.133 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750845785; cv=none; b=iR6UdMvH1WYX+BWxnj8s+V8l2wcdXqUEFdxSaYi5f4B/GQBivlqql0FwidWay/4wEpSmP4lvAdlVujKXYdW0Ae/75wvUpPjQFwQqs83piO+xqL5NzcYEUGIyITll77xgjLFBlvYDV7HXpNfiaIZOVlEcMZ6AwECSUFvuVDUVaUo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750845785; c=relaxed/simple; bh=Kk9QuwXeqHsh5m+WdFG3yfrYUrQof67Fj8u6Es2IJUc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=nQtax+leRoGEC+DLNDGHJcbSdImjv70GGkVV2aTKoC3IYNsZH9ncAfpUfJy+rSdV/ymB3QG2RiXtk9ID3pkkbwwSyj1DDpXNld1OSLYkkImxqQURT0PZXqyYQOqK+evTAK+key42kSFS/pfXaQfjVdoXTrcewIXysV3HxyBiuYg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=paYKpYi2; arc=none smtp.client-ip=115.124.30.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="paYKpYi2" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1750845774; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=Q2xEIG12L/x5PKgfFpP7sX4Fq2b51V9mENpLnQ81O2w=; b=paYKpYi2JaFFOB46wcSbB4cLOnfUxbJAx7hy9KMf7ZiE853LJKjz7rJNha5xycWj7gIfQdU9KM5IQRGmS42JZYj4MJ9F7EDTbF9qVAyHTSYeZNewKiBGfKTqLlE1WAw/W2KwQjKga0pwqwBlng8zNF2XZJ+kh1bpupxU2RyRn3w= Received: from 30.74.144.110(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0WewhO11_1750845771 cluster:ay36) by smtp.aliyun-inc.com; Wed, 25 Jun 2025 18:02:53 +0800 Message-ID: <37fc3553-0d5a-4bdc-b473-cd740d47598e@linux.alibaba.com> Date: Wed, 25 Jun 2025 18:02:51 +0800 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 v4 0/2] fix MADV_COLLAPSE issue if THP settings are disabled To: Lorenzo Stoakes Cc: David Hildenbrand , Hugh Dickins , akpm@linux-foundation.org, ziy@nvidia.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, zokeefe@google.com, shy828301@gmail.com, usamaarif642@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <75c02dbf-4189-958d-515e-fa80bb2187fc@google.com> <008ec97f-3b33-4b47-a112-9cef8c1d7f58@redhat.com> <01d679f2-fd64-472c-b9dc-ebe87268ce82@lucifer.local> <23b8ad10-cd1f-45df-a25c-78d01c8af44f@redhat.com> From: Baolin Wang In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2025/6/25 17:31, Lorenzo Stoakes wrote: > On Wed, Jun 25, 2025 at 04:52:03PM +0800, Baolin Wang wrote: >> >> >> On 2025/6/25 16:37, Lorenzo Stoakes wrote: >>> Yeah maybe the best way is to just have another tunable for this? >>> >>> /sys/kernel/mm/transparent_hugepage/disable_collapse perhaps? >>> >>> What do you think Hugh, Baolin? >> >> I think it's not necessary to find a way to disable madvise_collapse. >> Essentially, it's a conflict between the semantics of madvise_collapse and >> the '/sys/kernel/mm/transparent_hugepage/enabled' interface. We should reach >> a consensus on the semantics first: >> >> Semantic 1: madv_collapse() should ignore any THP system settings, meaning >> we need to update the 'never' semantics in >> '/sys/kernel/mm/transparent_hugepage/enabled', which would only disable page >> fault and khugepaged, not including madvise_collapse. If we agree on this, >> then the 'never' for per-sized mTHP would have the same semantics, i.e., >> when I set 64K mTHP to 'always' and 2M mTHP to 'never', madvise_collapse >> would still allow the collapse of 2M THP. We should document this clearly in >> case users still want 64K mTHP from madvise_collapse. > > Right yeah, I mean this is in effect how things are now. So the task is > documentation. > >> >> >> Semantic 2: madv_collapse() needs to respect THP system settings, which is >> what my patch does. Never means never, and we would need to update the >> documentation of madv_collapse() to make it clearer. > > Yes, and indeed this is the choice. > > I think, as David said, it comes down to whether we have a legit use case that > truly relies on this. > >>> (One side note on PMD-sized MADV_COLLAPSE - this is basically completely >>> useless for 64 KB page size arm64 systems where PMD's are 512 MB :) >>> >>> Thoughts Baolin? >> >> We should not collapse 512MB THP on 64K pagesize kernel. So seems >> madv_collapse() can not work on 64K pagesize kernel. > > Well I don't think anything would prevent this now right? So MADV_COLLAPSE is > pretty problematic on 64K pagesize kernels in general. Yes, I don't mean it will prevent madvise_collapse(), just as you said that it could be problematic (it's horrible to try to collapse 512MB). > Anyway that's maybe a problem for another time :) Yeah, should consider mTHP-compatible MADV_COLLAPSE.