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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, 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 E5DDBC433DF for ; Fri, 15 May 2020 18:37:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BFC512070A for ; Fri, 15 May 2020 18:37:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HG4DgOUI" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726247AbgEOShI (ORCPT ); Fri, 15 May 2020 14:37:08 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:42145 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726227AbgEOShI (ORCPT ); Fri, 15 May 2020 14:37:08 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589567826; 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=iS2BZEZhYnHE5mi0Tf0cUE2Vbxbk8ajM07NE1jWnMoA=; b=HG4DgOUIcBLWUcwjmssNuVyPsSwcepB0oWK4PhVdXwZyMbrSSKtpzCSiA4EFK5+ALHrZI9 zQQ9kY6p+UlHaLjRo72vi/3HJWWvkDp8leJqL4LioG6yi9G5qIl1Eks3LUdS+cRckv663a vgc4H9vPChxswmC8Tlu667GqHf7PuFs= 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-466-Zy5vaWusMkeTl9eED5XF2g-1; Fri, 15 May 2020 14:37:04 -0400 X-MC-Unique: Zy5vaWusMkeTl9eED5XF2g-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9B7B480B73A; Fri, 15 May 2020 18:37:03 +0000 (UTC) Received: from work-vm (ovpn-114-149.ams2.redhat.com [10.36.114.149]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0F3725C3E7; Fri, 15 May 2020 18:36:54 +0000 (UTC) Date: Fri, 15 May 2020 19:36:52 +0100 From: "Dr. David Alan Gilbert" To: David Hildenbrand Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, qemu-s390x@nongnu.org, Richard Henderson , Paolo Bonzini , Eduardo Habkost , "Michael S . Tsirkin" , Juan Quintela Subject: Re: [PATCH v1 07/17] migration/rdma: Use ram_block_discard_set_broken() Message-ID: <20200515183652.GM2954@work-vm> References: <20200506094948.76388-1-david@redhat.com> <20200506094948.76388-8-david@redhat.com> <20200515124501.GE2954@work-vm> <96a58e88-2629-f2ee-5884-38d11e571548@redhat.com> <20200515175105.GL2954@work-vm> <1cac6cb0-7804-bab2-4ecf-044c369c1135@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1cac6cb0-7804-bab2-4ecf-044c369c1135@redhat.com> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org * David Hildenbrand (david@redhat.com) wrote: > On 15.05.20 19:51, Dr. David Alan Gilbert wrote: > > * David Hildenbrand (david@redhat.com) wrote: > >> On 15.05.20 14:45, Dr. David Alan Gilbert wrote: > >>> * David Hildenbrand (david@redhat.com) wrote: > >>>> RDMA will pin all guest memory (as documented in docs/rdma.txt). We want > >>>> to mark RAM block discards to be broken - however, to keep it simple > >>>> use ram_block_discard_is_required() instead of inhibiting. > >>> > >>> Should this be dependent on whether rdma->pin_all is set? > >>> Even with !pin_all some will be pinned at any given time > >>> (when it's registered with the rdma stack). > >> > >> Do you know how much memory this is? Is such memory only temporarily pinned? > > > > With pin_all not set, only a subset of memory, I think multiple 1MB > > chunks, are pinned at any one time. > > > >> At least with special-cases of vfio, it's acceptable if some memory is > >> temporarily pinned - we assume it's only the working set of the driver, > >> which guests will not inflate as long as they don't want to shoot > >> themselves in the foot. > >> > >> This here sounds like the guest does not know the pinned memory is > >> special, right? > > > > Right - for RDMA it's all of memory that's being transferred, and the > > guest doesn't see when each part is transferred. > > > Okay, so all memory will eventually be pinned, just not at the same > time, correct? > > I think this implies that any memory that was previously discarded will > be backed my new pages, meaning we will consume more memory than intended. > > If so, always disabling discarding of RAM seems to be the right thing to do. Yeh that's probably true, although there's a check for 'buffer_is_zero' in the !rdma->pin_all case, if the entire area is zero (or probably if unmapped) then it sends a notification rather than registering; see qemu_rdma_write_one and search for 'This chunk has not yet been registered, so first check to see' Dave > > -- > Thanks, > > David / dhildenb -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK