From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 68614606A336 for ; Wed, 11 Nov 2026 12:37:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=hP3/mF1X Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1xR7Ze-0002kX-Vt; Wed, 11 Nov 2026 07:37:11 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1xR7ZX-0002is-De for qemu-devel@nongnu.org; Wed, 11 Nov 2026 07:37:04 -0500 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 1xR7ZV-0001Oc-El for qemu-devel@nongnu.org; Wed, 11 Nov 2026 07:37:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1794400620; 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: in-reply-to:in-reply-to:references:references; bh=sMfrvqWAxBU6yHWowUHPzvt+Iy+Os8eJPAwLpmC08hM=; b=hP3/mF1Xa4FBTTgvYEA34dEAONqgkZzcopF8O0bvNOrxv78CYsJGFPQynT5EXjq/2Dc1g2 xHm7PuctnQXV3Dbf0TnbIeNNPPLjLd886+SP3gMH2hxuaPImN5ly3Ds8APQeoOCjsfq6VU J/L5OLw6zdbbLmeqdkNsPyZpoQz6RRk= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-588-WUdK_FO0NDWDcv8bTpWbhw-1; Wed, 11 Nov 2026 07:36:56 -0500 X-MC-Unique: WUdK_FO0NDWDcv8bTpWbhw-1 X-Mimecast-MFC-AGG-ID: WUdK_FO0NDWDcv8bTpWbhw_1794400615 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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 mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A6BDA1800BEE; Wed, 11 Nov 2026 12:36:55 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.7]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 49D5B1956086; Wed, 11 Nov 2026 12:36:55 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 03A9221E6A04; Wed, 11 Nov 2026 13:36:53 +0100 (CET) From: Markus Armbruster To: qemu-devel@nongnu.org Cc: david@kernel.org, imammedo@redhat.com, berrange@redhat.com, peter.maydell@linaro.org Subject: Use of qemu_memfd_check() in system/physmem.c (was: [PATCH 0/7] backends/hostmem: Make QOM types and properties unconditional) In-Reply-To: <20261111122342.634370-1-armbru@redhat.com> (Markus Armbruster's message of "Wed, 11 Nov 2026 13:23:35 +0100") References: <20261111122342.634370-1-armbru@redhat.com> Date: Wed, 11 Nov 2026 13:36:52 +0100 Message-ID: <87qzgrv923.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-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: 2.1.29 Precedence: list List-Id: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 After this series, there is just one use of qemu_memfd_check() outside tests/ left: /* * Create MAP_SHARED RAMBlocks by mmap'ing a file descriptor, so it can be * shared with another process if CPR is being used. Use memfd if available * because it has no size limits, else use POSIX shm. */ static int qemu_ram_get_shared_fd(const char *name, bool *reused, Error **errp) { int fd = cpr_find_fd(name, 0); if (fd >= 0) { *reused = true; return fd; } ---> if (qemu_memfd_check(0)) { fd = qemu_memfd_create(name, 0, 0, 0, 0, errp); } else { fd = qemu_shm_alloc(0, errp); } if (fd >= 0 && !cpr_save_fd(name, 0, fd, errp)) { close(fd); fd = -1; } *reused = false; return fd; } We test whether memfd_create("test", MFD_CLOEXEC) succeeds to decide whether to use memfd_create(name, MFD_CLOEXEC) or POSIX shared memory. This is anti-pattern "predict which method should succed, then try that method". A better way is "try methods one after the other until one succeeds". Doing that here would require refactoring, because both qemu_memfd_create() and qemu_shm_alloc() do additional work that can also fail: ftruncate(). Doubt: qemu_memfd_create(0) caches its result[*]. Is that a mere optimization, or does something depend on "we use the same method always"? I don't plan to mess with this myself, I'm merely reporting something I found. [*] Only when the argument is 0.