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 8247D313272; Mon, 15 Jun 2026 18:24:55 +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=1781547896; cv=none; b=tRDl25SKRbLWt3DXQcdZWb/E2Kyb9xts1F757lr+3s4/ldmA+L76dIsybYNxuD6SfO+rDYuuIpGmTvfRKAvKmeNcS0jhrsX4xAuSDwIt2N4peHklfsq+qUWpMRdLk1JjGPkCot8k+bqvdA8CccHcyDleOH3FM5LF1hNZChFAs5Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781547896; c=relaxed/simple; bh=ueNYBkutiotF74POMTtciy1V50284ZCJj6e0EPF4ke0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=cVyarTed5BL3xd//u3jE+IrEATrUME6u+vKrQTYcN3x02VD9vAqBggxwLHW4Gt+qLxpU+BxEiQSE1DxujZXAFDLpa9fCcXGBdcZejLT9QS6DSVDOe8Ke/0RIFsvwLA5hAGq0eL6KwJMivl9jQlYi9bjll/xfjgSDr0bMJEwKx6A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eVaG8kfL; 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="eVaG8kfL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 80DF61F000E9; Mon, 15 Jun 2026 18:24:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781547895; bh=mu5V74iBeLUiBZiHPfWTe+e5iOSZcOAhU8BVXT1sRXw=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=eVaG8kfLV9DtJAz7uhfREk2guLdhwh8/uI9hktLJDiQ/z0TzJpQomwX3WesihLE3Q GdFirdffE7riVT4qQDma9hTkaF+nJoM8ObXbcLnHse22JKGbUWDU6PMq2kL4nCL3fO LR0EWSN6SQajXLDntOEgr3pQP9TVCF5jBrLTOG1PwXnIQmT9hKsxd4MHtYVAiGPrS5 xVMkW75kkveRYxHzPrsweixu/7MqJWdb7VCpSeS0u4+xiM1mmtdpksH5QC2qa7N6Vc qw2MFtx96J6hDf9mGNCeMtG/DXaz7pcoHUpIX7jn211ZnkkNQqx5DAsTdFMxBIt2nx SoVkS/7VVzQow== Message-ID: <1e77f24d-315a-4f68-a109-2f4520343c0c@kernel.org> Date: Mon, 15 Jun 2026 20:24:44 +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 RFC 0/3] KVM: guest_memfd: folio migration for non-confidential VMs To: Sean Christopherson , Alexandru Elisei Cc: Shivank Garg , "Matthew Wilcox (Oracle)" , Jan Kara , Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Johannes Weiner , Zi Yan , Matthew Brost , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Paolo Bonzini , Shuah Khan , Chao Peng , Nikunj A Dadhania , Ira Weiny , Michael Roth , Pankaj Gupta , Ackerley Tng , Fuad Tabba , Vishal Annapurve , Nikita Kalyazin , Patrick Roy , Pratik Sampat , Ashish Kalra , linux-fsdevel@vger.kernel.org, linux-coco@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20260611-shivank-gmem-migrate-v1-0-2d266bfc6f95@amd.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/15/26 19:39, Sean Christopherson wrote: > On Mon, Jun 15, 2026, Alexandru Elisei wrote: >> Hi, >> >> On Mon, Jun 15, 2026 at 11:43:14AM +0100, Alexandru Elisei wrote: >>> Hi, >>> >>> >>> I always thought that one of the nice things about using guest_memfd as a >>> memory backend, as opposed to host userspace mappings, is that the host >>> cannot unmap VM memory because of KSM, automatic NUMA balancing, hugepage >>> collapse, compaction, etc, acting on the host userspace mapping of the >>> VM memory, and outside of the VMM's or KVM's control. > > +1000. It's not just "nice to have", it's a core design principle of guest_memfd. Right, and I raised in the guest_memfd call also the rough idea of Alexandru's use case of having non-movable guest_memfd pages such that we can support use cases where we can hopefully guarantee that a stage-2 mapping will not just randomly go away. > >>> I think it would be useful to preserve this behaviour, even in the absence >>> of confidential VMs (i.e, guest_memfd file descriptor created with >>> GUEST_MEMFD_FLAG_MMAP). >> >> Just to be clear, I was thinking that it might be useful for both >> behaviours to exist (migratable and non-migratable) for non-confidential >> VMs, and allow KVM or userspace to decide which they prefer for a >> guest_memfd. > > For the purposes of this discussion, we should separate the physical act of > migrating pages from the features that trigger migration. As I said in last week's > guest-memfd call, I am a-ok with supporting page migration as a mechanism, but I > am dead set against supporting NUMA balancing, KSM, LRU-based swap/reclaim, and > anything else that goes against the goal of guest-first memory. Right. Page migration for supporting ZONE_MOVABLE/CMA, compaction, memory offlining, virtio-mem and possibly some collapse mechanism if we were to support THP of some sorts in guest_memfd would are all reasonable. As soon as we mix in access/lru semantics, we're going into the wrong direction. Fortunately KSM is anon-only and not even worth a rant here :) -- Cheers, David