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 X-Spam-Level: X-Spam-Status: No, score=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D137C433DB for ; Thu, 11 Mar 2021 17:21:07 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 5D16564FE7 for ; Thu, 11 Mar 2021 17:21:06 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5D16564FE7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:38930 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lKOze-0007TG-3A for qemu-devel@archiver.kernel.org; Thu, 11 Mar 2021 12:21:03 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:33536) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lKOup-0003cM-CN for qemu-devel@nongnu.org; Thu, 11 Mar 2021 12:16:04 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:58792) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lKOum-0007WQ-OD for qemu-devel@nongnu.org; Thu, 11 Mar 2021 12:16:02 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615482958; 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=QHyXD3TBjIQnB0dnXUHBp/jvnB0RiSuLtqAkYhLHSZE=; b=ae8d/if3PLNu77eeCuhR6rkd2pHB1P9X3uHZA74nJmMBgbDt2UvpOwGFWk7eT1l0SZwgIv zcW0T+KjUdAbPGZktPLq6Vt0Hf/d+UayJBlwlHUUL8xo7iHab7msoG+mBr6jIV9NO5B64G NXqQauhaaK1B5yBjTcPUIDlMNmydYS0= 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-420-_T_GfGNWM-CWKsIVZSyV9A-1; Thu, 11 Mar 2021 12:15:56 -0500 X-MC-Unique: _T_GfGNWM-CWKsIVZSyV9A-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2F6078064C9; Thu, 11 Mar 2021 17:15:55 +0000 (UTC) Received: from [10.36.115.26] (ovpn-115-26.ams2.redhat.com [10.36.115.26]) by smtp.corp.redhat.com (Postfix) with ESMTP id C491319705; Thu, 11 Mar 2021 17:15:16 +0000 (UTC) Subject: Re: [PATCH v3 02/12] softmmu/physmem: Fix ram_block_discard_range() to handle shared anonymous memory To: Peter Xu References: <20210308150600.14440-1-david@redhat.com> <20210308150600.14440-3-david@redhat.com> <20210311171153.GF194839@xz-x1> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <26d2c57a-971d-3abd-6ec3-1a38fb47a398@redhat.com> Date: Thu, 11 Mar 2021 18:15:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210311171153.GF194839@xz-x1> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, 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: Marcel Apfelbaum , Cornelia Huck , Eduardo Habkost , "Michael S. Tsirkin" , Stefan Weil , Murilo Opsfelder Araujo , Richard Henderson , "Dr. David Alan Gilbert" , Juan Quintela , qemu-devel@nongnu.org, Halil Pasic , Christian Borntraeger , Greg Kurz , Stefan Hajnoczi , Igor Mammedov , Thomas Huth , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Igor Kotrasinski Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 11.03.21 18:11, Peter Xu wrote: > On Thu, Mar 11, 2021 at 05:45:46PM +0100, David Hildenbrand wrote: >> On 11.03.21 17:39, Dr. David Alan Gilbert wrote: >>> * David Hildenbrand (david@redhat.com) wrote: >>>> We can create shared anonymous memory via >>>> "-object memory-backend-ram,share=on,..." >>>> which is, for example, required by PVRDMA for mremap() to work. >>>> >>>> Shared anonymous memory is weird, though. Instead of MADV_DONTNEED, we >>>> have to use MADV_REMOVE. MADV_DONTNEED fails silently and does nothing. >>> >>> OK, I wonder how stable these rules are; is it defined anywhere that >>> it's required? >>> >> >> I had a look at the Linux implementation: it's essentially shmem ... but we >> don't have an fd exposed, so we cannot use fallocate() ... :) >> >> MADV_REMOVE documents (man): >> >> "In the initial implementation, only tmpfs(5) was supported MADV_REMOVE; but >> since Linux 3.5, any filesystem which supports the fallocate(2) >> FALLOC_FL_PUNCH_HOLE mode also supports MADV_REMOVE." > > Hmm, I see that MADV_DONTNEED will still tear down all mappings even for > anonymous shmem.. what did I miss? Where did you see that? > MADV_DONTNEED only invalidates private copies in the pagecache. It's essentially useless for any kind of shared mappings. (I am 99.9% sure that we can replace fallocate()+MADV_DONTNEED by fallocate() for fd-based shared mappings, but that's a different story) -- Thanks, David / dhildenb