Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BCDBFC61D9D for ; Mon, 23 Jan 2023 14:05:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232146AbjAWOFn (ORCPT ); Mon, 23 Jan 2023 09:05:43 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53894 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231977AbjAWOFk (ORCPT ); Mon, 23 Jan 2023 09:05:40 -0500 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1FA9840F2; Mon, 23 Jan 2023 06:05:39 -0800 (PST) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id B4712339C7; Mon, 23 Jan 2023 14:05:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1674482737; h=from:from:reply-to: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=TboMeQ08DPimFEa6aGsq7T2+45CHDly2y+c6BUvgQZk=; b=Ohw7JlX3WCQz37Vbj1oIjlnjYp5zGfZW6t2Ldf4VxPph/WnY/v38NDh/e4hQhfkYr0L2nM tTDy+o417yh7yO/LPXj1Z8n7gftKOHJarxHrklKDBQus+7cmh+qEymeZvwvGa3iSoerIoa I9j84jts3ZCF0osIEYwGrzJjD6qJmeM= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1674482737; h=from:from:reply-to: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=TboMeQ08DPimFEa6aGsq7T2+45CHDly2y+c6BUvgQZk=; b=sUrDJ3R05y4rVwKZW1U5xV5ItVofPAnrWqkqedb0xLRv5AUbA46qNL6KwfAMuDVn1ihKfs SrGGxk/QC3zf4NAQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id AE567134F5; Mon, 23 Jan 2023 14:05:36 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id cjdAKTCUzmN6IwAAMHmgww (envelope-from ); Mon, 23 Jan 2023 14:05:36 +0000 Message-ID: <010a330c-a4d5-9c1a-3212-f9107d1c5f4e@suse.cz> Date: Mon, 23 Jan 2023 15:03:45 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.1 Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory Content-Language: en-US To: "Huang, Kai" , "chao.p.peng@linux.intel.com" Cc: "tglx@linutronix.de" , "linux-arch@vger.kernel.org" , "kvm@vger.kernel.org" , "jmattson@google.com" , "Hocko, Michal" , "pbonzini@redhat.com" , "ak@linux.intel.com" , "Lutomirski, Andy" , "linux-fsdevel@vger.kernel.org" , "tabba@google.com" , "david@redhat.com" , "michael.roth@amd.com" , "kirill.shutemov@linux.intel.com" , "corbet@lwn.net" , "qemu-devel@nongnu.org" , "dhildenb@redhat.com" , "bfields@fieldses.org" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" , "bp@alien8.de" , "ddutile@redhat.com" , "rppt@kernel.org" , "shuah@kernel.org" , "vkuznets@redhat.com" , "mail@maciej.szmigiero.name" , "naoya.horiguchi@nec.com" , "qperret@google.com" , "arnd@arndb.de" , "linux-api@vger.kernel.org" , "yu.c.zhang@linux.intel.com" , "Christopherson,, Sean" , "wanpengli@tencent.com" , "vannapurve@google.com" , "hughd@google.com" , "aarcange@redhat.com" , "mingo@redhat.com" , "hpa@zytor.com" , "Nakajima, Jun" , "jlayton@kernel.org" , "joro@8bytes.org" , "linux-mm@kvack.org" , "Wang, Wei W" , "steven.price@arm.com" , "linux-doc@vger.kernel.org" , "Hansen, Dave" , "akpm@linux-foundation.org" , "linmiaohe@huawei.com" References: <20221202061347.1070246-1-chao.p.peng@linux.intel.com> <20221202061347.1070246-2-chao.p.peng@linux.intel.com> <5c6e2e516f19b0a030eae9bf073d555c57ca1f21.camel@intel.com> <20221219075313.GB1691829@chaop.bj.intel.com> <20221220072228.GA1724933@chaop.bj.intel.com> <126046ce506df070d57e6fe5ab9c92cdaf4cf9b7.camel@intel.com> <20221221133905.GA1766136@chaop.bj.intel.com> From: Vlastimil Babka In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/22/22 01:37, Huang, Kai wrote: >>> I argue that this page pinning (or page migration prevention) is not >>> tied to where the page comes from, instead related to how the page will >>> be used. Whether the page is restrictedmem backed or GUP() backed, once >>> it's used by current version of TDX then the page pinning is needed. So >>> such page migration prevention is really TDX thing, even not KVM generic >>> thing (that's why I think we don't need change the existing logic of >>> kvm_release_pfn_clean()).  >>> > This essentially boils down to who "owns" page migration handling, and sadly, > page migration is kinda "owned" by the core-kernel, i.e. KVM cannot handle page > migration by itself -- it's just a passive receiver. > > For normal pages, page migration is totally done by the core-kernel (i.e. it > unmaps page from VMA, allocates a new page, and uses migrate_pape() or a_ops- >> migrate_page() to actually migrate the page). > In the sense of TDX, conceptually it should be done in the same way. The more > important thing is: yes KVM can use get_page() to prevent page migration, but > when KVM wants to support it, KVM cannot just remove get_page(), as the core- > kernel will still just do migrate_page() which won't work for TDX (given > restricted_memfd doesn't have a_ops->migrate_page() implemented). > > So I think the restricted_memfd filesystem should own page migration handling, > (i.e. by implementing a_ops->migrate_page() to either just reject page migration > or somehow support it). While this thread seems to be settled on refcounts already, just wanted to point out that it wouldn't be ideal to prevent migrations by a_ops->migrate_page() rejecting them. It would mean cputime wasted (i.e. by memory compaction) by isolating the pages for migration and then releasing them after the callback rejects it (at least we wouldn't waste time creating and undoing migration entries in the userspace page tables as there's no mmap). Elevated refcount on the other hand is detected very early in compaction so no isolation is attempted, so from that aspect it's optimal.