From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f197.google.com (mail-pg1-f197.google.com [209.85.215.197]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 472503644CA for ; Mon, 10 Aug 2026 23:45:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.197 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786405501; cv=none; b=r+QFBXZljW+cOJm/1kJ6MsY3Avm7TkLjn3s6ASL3PMuwjr8G8x/n8I7f1fBOHW+d9RWQqV2gE+iTKYmWj+HrgFqloe0BFhaiuokIDsMQBBrObtDbJlq4M07ZENWJtEJhygISSMec9PNHxdFvqIgvO/ZS21X3y3UuSkHyAre7/TQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786405501; c=relaxed/simple; bh=33plBrcKy5Ba/ieAnvSmeZoRK1u7IAJOuZUoYzX0htw=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=qBapsjbrf5tou6NYp+tJmzK5mdCJmqeeM9tp6+IRtM8bNKmtOyjobHgt2Wjvsk0Jd5862hk/N2KMoX1JraLb9yxWfV6AGmfXKec4/vWMvpzukHqBeMK+3FLQg5iNgvjWqRlc9vopXwSOmRRWNYi7sfQnu2VtY/6/ZkqonZ/MfUE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=o+6ZUIw1; arc=none smtp.client-ip=209.85.215.197 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="o+6ZUIw1" Received: by mail-pg1-f197.google.com with SMTP id 41be03b00d2f7-cb733fc5024so437972a12.2 for ; Mon, 10 Aug 2026 16:45:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786405499; x=1787010299; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=Lgf1v7ZyMRecW6GzYu5RKpdf3OAqJ1cTW2eoW0KSWKY=; b=o+6ZUIw1MbH/G+nWOFBorS6s4CZFDXy70d12/0joRZpcg7dgK+QMkownJHssYuzaUs 3kl9VADZAjGPeG+QspxwLyj5efMYKnOq4kJ5rsN2fuGTC4JPbWSrb/OQngVbADt1dCgV KxYm5MBHegA5nkhaXXYJPHeeJRGYCkwGi51mUb+gAY34inEHHrVAdXXl+ZA/cytpGlnt kh9E/xW7vXqy1oroDzglD0SjZ/XbY2pTmOzTKxgUrpZCv98LV9YDMqxhfqkz7bw/IhpG 8Ue43UPTNS3fTAb8IKD2F+ZBYq/ZdS7jEI9zh0v9QpWN/1a27xkToUKAzX/vkbr0HKBF SgyQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786405499; x=1787010299; h=content-type: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:content-type; bh=Lgf1v7ZyMRecW6GzYu5RKpdf3OAqJ1cTW2eoW0KSWKY=; b=Gp8MeUNvnt+ssv+T3Bph0V4F8pnAGrLmZFweWYQj7gjz9zEvg3P6JHvb6/ZWhh+mAo +rDjyGIuBYuPs8JriH0T1UMpFU6POKhc9rTDNkqcCPFKlt4mkMApC37AsZVUSxhudJac 03BTH8zFtPmc3EeuYAYVGERYoSeQ7WgAK+bVsZjG/sJPKysLYLAKZHor2XEuYhrHMiLN +/dEoc06evoR9ofUXo16FmgQUoiQGx2WhTJ/fNT+61+SHWwp0R+k/eXsF8+FxmBjOKqn 2tAa7QS0n8Lrp6GJUWSUtpIbRqLw4UcVBzevl4R5L+Onm7oKsj1ZfvHW+Zr8GSQsGrnB 1qIg== X-Forwarded-Encrypted: i=1; AHgh+Rrnzg7kZUWAOplb6wOQZixxwhMNeJKoYmWorgVCiRRDOWZRqSAzvJ9zgbUHECsDNGKOdunUT/FUnKuMr7xPAGc=@vger.kernel.org X-Gm-Message-State: AOJu0Yw1sy5UZyo9Gi+vFeya8v4gAes1ZfDm74gvJ4EF9WocuQlqhASQ ASitf2LvyiKwWnXF4I+EqFoGuiiYFRPCeYOwzOG1aIH3B5ugBO1pLa0THSkdgp8bEcrxq6a6TYE oGP1epQ== X-Received: from pggg21.prod.google.com ([2002:a63:2015:0:b0:c9b:bdf2:2507]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:178a:b0:845:e97d:447e with SMTP id d2e1a72fcca58-84f2e040170mr49815116b3a.26.1786405499340; Mon, 10 Aug 2026 16:44:59 -0700 (PDT) Date: Mon, 10 Aug 2026 16:44:58 -0700 In-Reply-To: <20260728121138.1103610-8-tarunsahu@google.com> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260728121138.1103610-1-tarunsahu@google.com> <20260728121138.1103610-8-tarunsahu@google.com> Message-ID: Subject: Re: [PATCH v4 07/11] KVM: guest_memfd: Add support for freezing mappings From: Sean Christopherson To: Tarun Sahu Cc: ackerleytng@google.com, fuad.tabba@linux.dev, Andrew Morton , dmatlack@google.com, Shuah Khan , Jonathan Corbet , david@redhat.com, Pasha Tatashin , Pratyush Yadav , sagis@google.com, Paolo Bonzini , Mike Rapoport , Alexander Graf , linux-kselftest@vger.kernel.org, andre.przywara@arm.com, michael.roth@amd.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, will@kernel.org, vannapurve@google.com, maz@kernel.org, fvdl@google.com, kvm@vger.kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev, alexandru.elisei@arm.com, skhawaja@google.com, aneesh.kumar@kernel.org, linux-doc@vger.kernel.org, David Hildenbrand , yan.y.zhao@intel.com, kexec@lists.infradead.org, suzuki.poulose@arm.com Content-Type: text/plain; charset="us-ascii" On Tue, Jul 28, 2026, Tarun Sahu wrote: > +/** > + * kvm_gmem_freeze - Freeze or unfreeze a guest_memfd inode mapping. > + * @inode: The guest_memfd inode. > + * @freeze: True to freeze, false to unfreeze. > + * > + * This API is used strictly during the live update / preservation transition > + * window to prevent host userspace and guest-side faults from making any > + * mapping modifications (such as fallocate or page fault allocation) > + * to the guest_memfd page cache. > + * > + * Synchronization Strategy (Sleepable RCU): > + * To avoid high-contention VFS locks (like inode_lock or > + * filemap_invalidate_lock) on the vCPU page fault hot paths, this subsystem > + * implements a lightweight, system-wide Sleepable RCU (SRCU) mechanism > + * (`kvm_gmem_freeze_srcu`): > + * > + * Global vs. Per-Inode SRCU > + * ====================== > + * A single system-wide global static `srcu_struct` is used instead of a > + * per-inode SRCU structure to completely prevent unprivileged users from > + * exhausting the host's per-CPU memory allocator. This argument doesn't hold up given that each kvm structure has two SRCU structs.