From nobody Mon Feb 9 09:22:44 2026 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=fail; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1626860149474768.9104632948339; Wed, 21 Jul 2021 02:35:49 -0700 (PDT) Received: from localhost ([::1]:45856 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1m68do-0003kO-G1 for importer@patchew.org; Wed, 21 Jul 2021 05:35:48 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41500) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m68Wm-0008QO-08 for qemu-devel@nongnu.org; Wed, 21 Jul 2021 05:28:32 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:21418) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1m68Wi-0002GE-2K for qemu-devel@nongnu.org; Wed, 21 Jul 2021 05:28:30 -0400 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-37-5oKEpxroM12HfWropJc9Vg-1; Wed, 21 Jul 2021 05:28:26 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id F21C180259B; Wed, 21 Jul 2021 09:28:24 +0000 (UTC) Received: from t480s.redhat.com (ovpn-113-250.ams2.redhat.com [10.36.113.250]) by smtp.corp.redhat.com (Postfix) with ESMTP id F1D3F687D5; Wed, 21 Jul 2021 09:28:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1626859707; h=from:from: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; bh=KDHE5WeCEv+6agq7D2mw+/Z52QQ9LvAe20WYe2wi8BE=; b=CruxgFBLaGHeagK2juYFmk0OgGGuKdRX++NzN8+vhvmTW+ThaC8vMEMuTMrZitrvYGEl3Y mfZX0Js79gGxuoo/C1LW63egQzMxAMq3gdn4zXtOxNn5PUuoiyPWlfTcoNbpwpU753Khem Wk4TDy+pzgM0+ina1OtJovOnWMcCHNs= X-MC-Unique: 5oKEpxroM12HfWropJc9Vg-1 From: David Hildenbrand To: qemu-devel@nongnu.org Subject: [PATCH v2 6/6] migration/ram: Handle RAMBlocks with a RamDiscardManager on background snapshots Date: Wed, 21 Jul 2021 11:27:59 +0200 Message-Id: <20210721092759.21368-7-david@redhat.com> In-Reply-To: <20210721092759.21368-1-david@redhat.com> References: <20210721092759.21368-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=216.205.24.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.474, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , "Michael S. Tsirkin" , Pankaj Gupta , Juan Quintela , David Hildenbrand , "Dr. David Alan Gilbert" , Peter Xu , Marek Kedzierski , Alex Williamson , teawater , Paolo Bonzini , Andrey Gruzdev , Wei Yang Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) X-ZM-MESSAGEID: 1626860150146100001 Content-Type: text/plain; charset="utf-8" We already don't ever migrate memory that corresponds to discarded ranges as managed by a RamDiscardManager responsible for the mapped memory region of the RAMBlock. virtio-mem uses this mechanism to logically unplug parts of a RAMBlock. Right now, we still populate zeropages for the whole usable part of the RAMBlock, which is undesired because: 1. Even populating the shared zeropage will result in memory getting consumed for page tables. 2. Memory backends without a shared zeropage (like hugetlbfs and shmem) will populate an actual, fresh page, resulting in an unintended memory consumption. Discarded ("logically unplugged") parts have to remain discarded. As these pages are never part of the migration stream, there is no need to track modifications via userfaultfd WP reliably for these parts. Further, any writes to these ranges by the VM are invalid and the behavior is undefined. Note that Linux only supports userfaultfd WP on private anonymous memory for now, which usually results in the shared zeropage getting populated. The issue will become more relevant once userfaultfd WP supports shmem and hugetlb. Signed-off-by: David Hildenbrand Acked-by: Peter Xu --- migration/ram.c | 53 +++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 45 insertions(+), 8 deletions(-) diff --git a/migration/ram.c b/migration/ram.c index d7505f5368..75de936bd2 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -1612,6 +1612,28 @@ out: return ret; } =20 +static inline void populate_range(RAMBlock *block, hwaddr offset, hwaddr s= ize) +{ + char *ptr =3D (char *) block->host; + + for (; offset < size; offset +=3D qemu_real_host_page_size) { + char tmp =3D *(ptr + offset); + + /* Don't optimize the read out */ + asm volatile("" : "+r" (tmp)); + } +} + +static inline int populate_section(MemoryRegionSection *section, void *opa= que) +{ + const hwaddr size =3D int128_get64(section->size); + hwaddr offset =3D section->offset_within_region; + RAMBlock *block =3D section->mr->ram_block; + + populate_range(block, offset, size); + return 0; +} + /* * ram_block_populate_pages: populate memory in the RAM block by reading * an integer from the beginning of each page. @@ -1621,16 +1643,31 @@ out: * * @block: RAM block to populate */ -static void ram_block_populate_pages(RAMBlock *block) +static void ram_block_populate_pages(RAMBlock *rb) { - char *ptr =3D (char *) block->host; - - for (ram_addr_t offset =3D 0; offset < block->used_length; - offset +=3D qemu_real_host_page_size) { - char tmp =3D *(ptr + offset); + /* + * Skip populating all pages that fall into a discarded range as manag= ed by + * a RamDiscardManager responsible for the mapped memory region of the + * RAMBlock. Such discarded ("logically unplugged") parts of a RAMBlock + * must not get populated automatically. We don't have to track + * modifications via userfaultfd WP reliably, because these pages will + * not be part of the migration stream either way -- see + * ramblock_dirty_bitmap_exclude_discarded_pages(). + * + * Note: The result is only stable while migration (precopy/postcopy). + */ + if (rb->mr && memory_region_has_ram_discard_manager(rb->mr)) { + RamDiscardManager *rdm =3D memory_region_get_ram_discard_manager(r= b->mr); + MemoryRegionSection section =3D { + .mr =3D rb->mr, + .offset_within_region =3D 0, + .size =3D rb->mr->size, + }; =20 - /* Don't optimize the read out */ - asm volatile("" : "+r" (tmp)); + ram_discard_manager_replay_populated(rdm, §ion, + populate_section, NULL); + } else { + populate_range(rb, 0, qemu_ram_get_used_length(rb)); } } =20 --=20 2.31.1