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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 92F82D116F1 for ; Mon, 1 Dec 2025 09:12:53 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id EAFA36B0006; Mon, 1 Dec 2025 04:12:52 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E5FCE6B0092; Mon, 1 Dec 2025 04:12:52 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D9C826B0093; Mon, 1 Dec 2025 04:12:52 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id CBA426B0006 for ; Mon, 1 Dec 2025 04:12:52 -0500 (EST) Received: from smtpin26.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id E0BF151731 for ; Mon, 1 Dec 2025 09:12:49 +0000 (UTC) X-FDA: 84170337258.26.B918BF4 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf27.hostedemail.com (Postfix) with ESMTP id 0BB794000E for ; Mon, 1 Dec 2025 09:12:47 +0000 (UTC) Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=O8zdSJ62; spf=pass (imf27.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1764580368; h=from:from:sender: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:dkim-signature; bh=RYuwXzXCSNoZY/RhYfUUB9WrkA3ENFTb7WvxlwFcDD0=; b=SIiWGR51pgIxJDIkCba+6HHHoEQNHzl8bvbqBj4fFiM0Urgngtqyjm9pD+z51XBAJ8WnqI Mq8EPZG0n6rogwvIhcivLNhGu8ZOUDxYh1FOwkelMW6ISVrblUSzC+zGcb9leBtlZADFLY 2wJREyYWCMAJeBsD9WVyYG4f33Q8Qy8= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=O8zdSJ62; spf=pass (imf27.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1764580368; a=rsa-sha256; cv=none; b=I/2wQP0+4dC+wZ2/gkz5FBuyV4x6aiR6qH3UbsDK4uQ5ghWJGA9gN52mJMwLj3dJViKVv7 wjLcHiV5m2L6O3J8jnhJpkJzm33xVDVoEEuK07n3BTlsos1aDZUWHuyLFvMZ6nxREgMZnD biTlkH0XQ/pTKlMAwRxnbYcN0Ortwe4= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 0C76144479; Mon, 1 Dec 2025 09:12:47 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 48C80C4CEF1; Mon, 1 Dec 2025 09:12:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764580366; bh=InzzoRi4sMwF5UAjcjxn8h6bfGO56WHUkwSr4ysFVAs=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=O8zdSJ62lsZX1UqcRK56slWK1In8AZrXMfXAd2VdEbVk2Xgn+zXW6pigE42wp/BLx JOgSmGg2QpEtSdamdFLOsSOfx81Oh6q5uN4Wae0Mjs6thxYpfTIeR1AVP3hycf2kCk HhTGTeO+roQo6HTCW4AqTK+xOJtDMo+4GtudAnmSze9y0se0WAFD8eNAf/tW1lAxDT JEjoGXoJDZuHIKqwfVbjHImB2pJTw+MCNlFms74PVN1SX5QUUl0DXX53ZSg8d8nXJ1 6YPGx/UbSwjz1j4BmMeZJI59h4wO09tf7mo0KjfzD/30bvjTjfmavRhdSfcsb7aUqz R01CkzAKLSo9g== Message-ID: Date: Mon, 1 Dec 2025 10:12:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor mode To: Mike Rapoport , linux-mm@kvack.org Cc: Andrea Arcangeli , Andrew Morton , Axel Rasmussen , Baolin Wang , Hugh Dickins , James Houghton , "Liam R. Howlett" , Lorenzo Stoakes , Michal Hocko , Nikita Kalyazin , Paolo Bonzini , Peter Xu , Sean Christopherson , Shuah Khan , Suren Baghdasaryan , Vlastimil Babka , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org References: <20251130111812.699259-1-rppt@kernel.org> <20251130111812.699259-5-rppt@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <20251130111812.699259-5-rppt@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: risg8frw7omqusuwu6xqsoojez1k5ccb X-Rspam-User: X-Rspamd-Queue-Id: 0BB794000E X-Rspamd-Server: rspam09 X-HE-Tag: 1764580367-901521 X-HE-Meta: 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 B75gULez Kj4y/3sTH7MirTqtyFr5W7+5CGdMfdj84X2zK6vK79lN1h36YPA6NBTbUjDFE2PcaMxIkBV8UQyAK+4yEolbvwcacaCu1cmtBGUCT2sClGOCWCUl022JLwFO4npwbNGc3yPYTCRn6V2WF3Iq4U0lDFig6pQEtlhiEVLxizpZtL4BTpNx5xiJyM4J7V2T8yqn1uQFedRmg37lzcan3AdMOK+W4IiTKSabYQvL3sbIdt3/HIKRaksjqE643lX18Y4Gbbk4fogM+yb0+pdmKZ4hb/22KyC+P0ZbDflRf1xj57G9f6oiTL/KhWoBKfIFPREp/TmqWhvqRDfb0Dgh8rUO7MXmZVB4FYny4HOx7 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/30/25 12:18, Mike Rapoport wrote: > From: "Mike Rapoport (Microsoft)" > > userfaultfd notifications about minor page faults used for live migration > and snapshotting of VMs with memory backed by shared hugetlbfs or tmpfs > mappings as described in detail in commit 7677f7fd8be7 ("userfaultfd: add > minor fault registration mode"). > > To use the same mechanism for VMs that use guest_memfd to map their memory, > guest_memfd should support userfaultfd minor mode. > > Extend ->fault() method of guest_memfd with ability to notify core page > fault handler that a page fault requires handle_userfault(VM_UFFD_MINOR) to > complete and add implementation of ->get_folio_noalloc() to guest_memfd > vm_ops. > > Reviewed-by: Liam R. Howlett > Signed-off-by: Mike Rapoport (Microsoft) > --- > virt/kvm/guest_memfd.c | 33 ++++++++++++++++++++++++++++++++- > 1 file changed, 32 insertions(+), 1 deletion(-) > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index ffadc5ee8e04..dca6e373937b 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -4,6 +4,7 @@ > #include > #include > #include > +#include > > #include "kvm_mm.h" > > @@ -359,7 +360,15 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) > if (!((u64)inode->i_private & GUEST_MEMFD_FLAG_INIT_SHARED)) > return VM_FAULT_SIGBUS; > > - folio = kvm_gmem_get_folio(inode, vmf->pgoff); > + folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff); > + if (!IS_ERR_OR_NULL(folio) && userfaultfd_minor(vmf->vma)) { Can we ever get NULL here? > + ret = VM_FAULT_UFFD_MINOR; > + goto out_folio; > + } > + > + if (PTR_ERR(folio) == -ENOENT) > + folio = kvm_gmem_get_folio(inode, vmf->pgoff); Was briefly wondering what the performance impact of that two-step approach is (two lookups in case we have to create it IIUC) Wouldn't it be better to limit it to the userfaultfd_minor(vmf->vma) case? if (userfaultfd_minor(vmf->vma)) { folio = filemap_lock_folio(inode->i_mapping, vmf->pgoff); if (!IS_ERR(folio)) { ret = VM_FAULT_UFFD_MINOR; goto out_folio; } } else { folio = kvm_gmem_get_folio(inode, vmf->pgoff); } if (IS_ERR(folio)) { ... -- Cheers David