From mboxrd@z Thu Jan 1 00:00:00 1970 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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 92A9EC4167D for ; Wed, 1 Nov 2023 13:42:45 +0000 (UTC) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=jRcODWVN; dkim-atps=neutral Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4SL7VX1jCjz3cST for ; Thu, 2 Nov 2023 00:42:44 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=jRcODWVN; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=flex--seanjc.bounces.google.com (client-ip=2607:f8b0:4864:20::114a; helo=mail-yw1-x114a.google.com; envelope-from=3mlvczqykdh8vhdqmfjrrjoh.frpolqx0ssf-ghyolvwv.r2odev.ruj@flex--seanjc.bounces.google.com; receiver=lists.ozlabs.org) Received: from mail-yw1-x114a.google.com (mail-yw1-x114a.google.com [IPv6:2607:f8b0:4864:20::114a]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4SL7TX027xz2xl6 for ; Thu, 2 Nov 2023 00:41:50 +1100 (AEDT) Received: by mail-yw1-x114a.google.com with SMTP id 00721157ae682-5a8ee6a1801so80240987b3.3 for ; Wed, 01 Nov 2023 06:41:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698846107; x=1699450907; darn=lists.ozlabs.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=nFf6aEaGwk5ej5JezQy0IgMN87WPAcTCXufwbiE3fug=; b=jRcODWVNf4HPt/va7ieM1Oezre/1ZPdWH79NhBnmGGB0VJmJ3hpXQvFc6coh8/MsQL d2SU7kvYsXJLZag7WMVg1YXuLvXHUtOogOmTMKcwFemUEa3qujknBlcHhgYgExmILi5T ce0DaHXHXVME3BI2k7ph5v8wy7Qt/OMS02Tc+BwAihA4v3HkTdOx8x3HXQGzPELXFqo7 9rOk30vPGydMWBqkK3IrX29JukAqCgKLTATyDYnZnsdSd+dQEbPTxF7LS7jxm7NIriX+ ETQX5uXCuilPmIUErywWyRr+g1zPYrFAWzHQoa+SyINAHUwePmvk7gv/gU6QQ0sGBpHY y7qg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698846107; x=1699450907; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=nFf6aEaGwk5ej5JezQy0IgMN87WPAcTCXufwbiE3fug=; b=srG2NrmsOiDtSsWL7VfR+CyCV05eOCmvbDs6G+gOC4LCB1WhrrbnaMXeCMccB37gGT GC3LxIwA8EN2rDQXjeyWM4ZNlsi9UaBiAtZ495Eq6yk5X6fpPVp+qOhmfn90wLhfEhqW wTEUE9vcVcfmhsloyG5XqvwzUl7RRSR7FLQg9MrHVH/55qhxjVM8ajK/GtLcTtKcscfx DBRblF88iBXrTySJdrn5cv/pXgEaEAnxwb1dqhswwKd+JmjsWiiNg0WxLa0TCzsm7bdx tKIst8wx33uCSmlar4fxOTVaJXSkwE6QBJ/dWWkUSPqml+H76S2mwyUTa62KVpBRhCYg 4rww== X-Gm-Message-State: AOJu0YwUyQTRYOdiahf4hxpkrPH3dnVwRDCfRrBk63dRZL5Cs2C9SrK1 zCvJ+pgDKZG+EVA0o/du4vd7MbXmj+c= X-Google-Smtp-Source: AGHT+IEsRM1yQnQClQeVqyoNLxT9cYgRny7PWezkHcY0wXTm/cdeLyJgnrb+5OZENHYJQKvQlFBt8yx8E+I= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a0d:e689:0:b0:5a7:bfcf:2cb8 with SMTP id p131-20020a0de689000000b005a7bfcf2cb8mr314827ywe.1.1698846106686; Wed, 01 Nov 2023 06:41:46 -0700 (PDT) Date: Wed, 1 Nov 2023 06:41:45 -0700 In-Reply-To: <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-18-seanjc@google.com> <7c0844d8-6f97-4904-a140-abeabeb552c1@intel.com> <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com> Message-ID: Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory From: Sean Christopherson To: Xiaoyao Li Content-Type: text/plain; charset="us-ascii" X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kvm@vger.kernel.org, David Hildenbrand , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Chao Peng , linux-riscv@lists.infradead.org, Isaku Yamahata , Marc Zyngier , Huacai Chen , "Matthew Wilcox \(Oracle\)" , Wang , Fuad Tabba , Yu Zhang , Maciej Szmigiero , Albert Ou , Vlastimil Babka , Michael Roth , Ackerley Tng , Alexander Viro , Paul Walmsley , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= , Isaku Yamahata , Christian Brauner , Quentin Perret , Liam Merwick , linux-mips@vger.kernel.org, Oliver Upton , David Matlack , Jarkko Sakkinen , Palmer Dabbelt , "Kirill A . Shutemov" , kvm-riscv@lists.infradead.org, Anup Patel , linux-fsdevel@vger.kernel.org, Paolo Bonzini , Andrew Morton , Vishal Annapurve , linuxppc-dev@lists.ozlabs.org, Xu Yilun , Anish Moorthy Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, Nov 01, 2023, Xiaoyao Li wrote: > On 10/31/2023 10:16 PM, Sean Christopherson wrote: > > On Tue, Oct 31, 2023, Xiaoyao Li wrote: > > > On 10/28/2023 2:21 AM, Sean Christopherson wrote: > > > > Extended guest_memfd to allow backing guest memory with transparent > > > > hugepages. Require userspace to opt-in via a flag even though there's no > > > > known/anticipated use case for forcing small pages as THP is optional, > > > > i.e. to avoid ending up in a situation where userspace is unaware that > > > > KVM can't provide hugepages. > > > > > > Personally, it seems not so "transparent" if requiring userspace to opt-in. > > > > > > People need to 1) check if the kernel built with TRANSPARENT_HUGEPAGE > > > support, or check is the sysfs of transparent hugepage exists; 2)get the > > > maximum support hugepage size 3) ensure the size satisfies the alignment; > > > before opt-in it. > > > > > > Even simpler, userspace can blindly try to create guest memfd with > > > transparent hugapage flag. If getting error, fallback to create without the > > > transparent hugepage flag. > > > > > > However, it doesn't look transparent to me. > > > > The "transparent" part is referring to the underlying kernel mechanism, it's not > > saying anything about the API. The "transparent" part of THP is that the kernel > > doesn't guarantee hugepages, i.e. whether or not hugepages are actually used is > > (mostly) transparent to userspace. > > > > Paolo also isn't the biggest fan[*], but there are also downsides to always > > allowing hugepages, e.g. silent failure due to lack of THP or unaligned size, > > and there's precedent in the form of MADV_HUGEPAGE. > > > > [*] https://lore.kernel.org/all/84a908ae-04c7-51c7-c9a8-119e1933a189@redhat.com > > But it's different than MADV_HUGEPAGE, in a way. Per my understanding, the > failure of MADV_HUGEPAGE is not fatal, user space can ignore it and > continue. > > However, the failure of KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is fatal, which leads > to failure of guest memfd creation. Failing KVM_CREATE_GUEST_MEMFD isn't truly fatal, it just requires different action from userspace, i.e. instead of ignoring the error, userspace could redo KVM_CREATE_GUEST_MEMFD with KVM_GUEST_MEMFD_ALLOW_HUGEPAGE=0. We could make the behavior more like MADV_HUGEPAGE, e.g. theoretically we could extend fadvise() with FADV_HUGEPAGE, or add a guest_memfd knob/ioctl() to let userspace provide advice/hints after creating a guest_memfd. But I suspect that guest_memfd would be the only user of FADV_HUGEPAGE, and IMO a post-creation hint is actually less desirable. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE will fail only if userspace didn't provide a compatible size or the kernel doesn't support THP. An incompatible size is likely a userspace bug, and for most setups that want to utilize guest_memfd, lack of THP support is likely a configuration bug. I.e. many/most uses *want* failures due to KVM_GUEST_MEMFD_ALLOW_HUGEPAGE to be fatal. > For current implementation, I think maybe KVM_GUEST_MEMFD_DESIRE_HUGEPAGE > fits better than KVM_GUEST_MEMFD_ALLOW_HUGEPAGE? or maybe *PREFER*? Why? Verbs like "prefer" and "desire" aren't a good fit IMO because they suggest the flag is a hint, and hints are usually best effort only, i.e. are ignored if there is a fundamental incompatibility. "Allow" isn't perfect, e.g. I would much prefer a straight KVM_GUEST_MEMFD_USE_HUGEPAGES or KVM_GUEST_MEMFD_HUGEPAGES flag, but I wanted the name to convey that KVM doesn't (yet) guarantee hugepages. I.e. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is stronger than a hint, but weaker than a requirement. And if/when KVM supports a dedicated memory pool of some kind, then we can add KVM_GUEST_MEMFD_REQUIRE_HUGEPAGE.