From patchwork Thu Jul 6 07:56:06 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 13303305 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.gnu.org (lists.gnu.org [209.51.188.17]) (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 DA5C1EB64DC for ; Thu, 6 Jul 2023 07:57:24 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qHJrD-0002Ht-5j; Thu, 06 Jul 2023 03:56:55 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qHJr2-0002F1-4O for qemu-devel@nongnu.org; Thu, 06 Jul 2023 03:56:45 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qHJqy-0007Lj-1i for qemu-devel@nongnu.org; Thu, 06 Jul 2023 03:56:41 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1688630199; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bB5am1/wievjqeDq9pWP6E7oZd7KTgSHM6XOUFzUNWk=; b=DipAsc16b2Bt6ARVgC/qzXRqyxLH6oIhUMjOndhnK4e0F2JfY56tARde0TJ6XQwt6CiYYH s0uy0tXCn6g1MDxFsCE1zjUI0nuFd+fX4YgEtVm9gybmXo5cGe3LVgE/WyWuKxQtgzWzMs NDkDMp/Yy31OnyV9Tf+j3QqjlAYeQ5g= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-270-RgvSpwvQPr20vwZkcWb6wA-1; Thu, 06 Jul 2023 03:56:35 -0400 X-MC-Unique: RgvSpwvQPr20vwZkcWb6wA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 43F2838117E7; Thu, 6 Jul 2023 07:56:35 +0000 (UTC) Received: from t14s.cit.tum.de (unknown [10.39.193.162]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4CC1318EB4; Thu, 6 Jul 2023 07:56:33 +0000 (UTC) From: David Hildenbrand To: qemu-devel@nongnu.org Cc: David Hildenbrand , "Michael S. Tsirkin" , Juan Quintela , Peter Xu , Leonardo Bras , Paolo Bonzini , =?utf-8?q?Philippe_Mathieu-Daud=C3=A9?= , Peng Tao , Mario Casquero Subject: [PATCH v2 1/4] softmmu/physmem: Warn with ram_block_discard_range() on MAP_PRIVATE file mapping Date: Thu, 6 Jul 2023 09:56:06 +0200 Message-ID: <20230706075612.67404-2-david@redhat.com> In-Reply-To: <20230706075612.67404-1-david@redhat.com> References: <20230706075612.67404-1-david@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org ram_block_discard_range() cannot possibly do the right thing in MAP_PRIVATE file mappings in the general case. To achieve the documented semantics, we also have to punch a hole into the file, possibly messing with other MAP_PRIVATE/MAP_SHARED mappings of such a file. For example, using VM templating -- see commit b17fbbe55cba ("migration: allow private destination ram with x-ignore-shared") -- in combination with any mechanism that relies on discarding of RAM is problematic. This includes: * Postcopy live migration * virtio-balloon inflation/deflation or free-page-reporting * virtio-mem So at least warn that there is something possibly dangerous is going on when using ram_block_discard_range() in these cases. Acked-by: Peter Xu Tested-by: Mario Casquero Signed-off-by: David Hildenbrand Reviewed-by: Juan Quintela --- softmmu/physmem.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/softmmu/physmem.c b/softmmu/physmem.c index bda475a719..4ee157bda4 100644 --- a/softmmu/physmem.c +++ b/softmmu/physmem.c @@ -3456,6 +3456,24 @@ int ram_block_discard_range(RAMBlock *rb, uint64_t start, size_t length) * so a userfault will trigger. */ #ifdef CONFIG_FALLOCATE_PUNCH_HOLE + /* + * We'll discard data from the actual file, even though we only + * have a MAP_PRIVATE mapping, possibly messing with other + * MAP_PRIVATE/MAP_SHARED mappings. There is no easy way to + * change that behavior whithout violating the promised + * semantics of ram_block_discard_range(). + * + * Only warn, because it work as long as nobody else uses that + * file. + */ + if (!qemu_ram_is_shared(rb)) { + warn_report_once("ram_block_discard_range: Discarding RAM" + " in private file mappings is possibly" + " dangerous, because it will modify the" + " underlying file and will affect other" + " users of the file"); + } + ret = fallocate(rb->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, start, length); if (ret) {