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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]:45170 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kkBr6-0001Lu-Op for qemu-devel@archiver.kernel.org; Tue, 01 Dec 2020 15:02:32 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:51822) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kkBpl-0000kv-FS for qemu-devel@nongnu.org; Tue, 01 Dec 2020 15:01:09 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:40651) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kkBpe-0000tR-Jv for qemu-devel@nongnu.org; Tue, 01 Dec 2020 15:01:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606852859; 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=2Xxe9ZSNt8nlXfpYxghkCByYa4R1uzrSJOXGB4MwP64=; b=etH9pLS9tJCmCBk6F0yLNQne5CG6923UwjPNE9Fw99fnrJJc2jUX6vI2n1T8kQ01Gxz2j7 1o4mUTHOFX656wulSn1EB1/LlD7+CcsMtj78vJqeuuyAy70RiUXD0g6qRcK2RSWcT02MOw 5UbEfZnAgB85Uo/HucDRwWU803UMLmw= 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-193-fUv4qNSHMV--Z5bPUTdjSQ-1; Tue, 01 Dec 2020 15:00:57 -0500 X-MC-Unique: fUv4qNSHMV--Z5bPUTdjSQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 82C5D858182; Tue, 1 Dec 2020 20:00:56 +0000 (UTC) Received: from work-vm (ovpn-115-1.ams2.redhat.com [10.36.115.1]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3D59B5B4A1; Tue, 1 Dec 2020 20:00:50 +0000 (UTC) Date: Tue, 1 Dec 2020 20:00:48 +0000 From: "Dr. David Alan Gilbert" To: Peter Xu Subject: Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots Message-ID: <20201201200048.GP4338@work-vm> References: <20201126151734.743849-1-andrey.gruzdev@virtuozzo.com> <20201201070820.GO105758@angien.pipo.sk> <20201201105300.GQ105758@angien.pipo.sk> <20201201185438.GE3277@xz-x1> MIME-Version: 1.0 In-Reply-To: <20201201185438.GE3277@xz-x1> User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=dgilbert@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, 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_H3=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: Andrea Arcangeli , Peter Krempa , Juan Quintela , qemu-devel@nongnu.org, Markus Armbruster , Paolo Bonzini , Den Lunev , Andrey Gruzdev Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Peter Xu (peterx@redhat.com) wrote: > On Tue, Dec 01, 2020 at 02:24:12PM +0300, Andrey Gruzdev wrote: > > On 01.12.2020 13:53, Peter Krempa wrote: > > > On Tue, Dec 01, 2020 at 11:42:18 +0300, Andrey Gruzdev wrote: > > > > On 01.12.2020 10:08, Peter Krempa wrote: > > > > > On Thu, Nov 26, 2020 at 18:17:28 +0300, Andrey Gruzdev via wrote: > > > > > > This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's > > > > > > [...] > > > > > > > > Note that in cases when qemu can't guarantee that the > > > > > background_snapshot feature will work it should not advertise it. We > > > > > need a way to check whether it's possible to use it, so we can replace > > > > > the existing --live flag with it rather than adding a new one and > > > > > shifting the problem of checking whether the feature works to the user. > > Would it be fine if libvirt just try the new way first anyways? Since if it > will fail, it'll fail right away on any unsupported memory types, then > logically the libvirt user may not even notice we've retried. > > Previously I thought it was enough, because so far the kernel does not have a > specific flag showing whether such type of memory is supported. But I don't > know whether it would be non-trivial for libvirt to retry like that. > > Another solution is to let qemu test the uffd ioctls right after QEMU memory > setup, so we know whether background/live snapshot is supported or not with > current memory backends. We should need to try this for every ramblock because > I think we can have different types across all the qemu ramblocks. I don't think we actually do that for postcopy; we do some checks like checking if we have any hugepages, and if so checking for it's flags. But note that we do tie it into migrate_caps_check to fail if you try and set the capability. > > > > > > > > > > > > > > Hi, > > > > > > > > May be you are using hugetlbfs as memory backend? > > > > > > Not exactly hugepages, but I had: > > > > > > > > > > > > > > > > > > which resulted into the following commandline to instantiate memory: > > > > > > -object memory-backend-file,id=pc.ram,mem-path=/var/lib/libvirt/qemu/ram/6-upstream-bj/pc.ram,share=yes,size=33554432000,host-nodes=0,policy=bind \ > > > > > > When I've removed it I got: > > > > > > -object memory-backend-ram,id=pc.ram,size=33554432000,host-nodes=0,policy=bind \ > > > > > > And the migration didn't fail in my quick test. I'll have a more > > > detailed look later, thanks for the pointer. > > > > > > > Yep, seems that current userfaultfd supports hugetlbfs and shared memory for > > missing pages but not for wr-protected.. > > Correct. Btw, I'm working on both of them recently. I have a testing kernel > branch, but I don't think it should affect our qemu work, though, since qemu > should do the same irrelevant of the memory type. We can just test with > anonymous memories, and as long as it works, it should work perfectly on all > the rest of backends (maybe even for other file-backed memory, more below). > > > > > > > I totally agree that we need somehow check that kernel and VM memory backend > > > > support the feature before one can enable the capability. > > > > Need to think about that.. > > > > > > Definitely. Also note that memory backed by memory-backend-file will be > > > more and more common, for cases such as virtiofs DAX sharing and > > > similar. > > > > > > > I see.. That needs support from kernel side, so far 'background-snapshots' > > are incompatible with memory-backend-file sharing. > > Yes. So as mentioned, shmem/hugetlbfs should be WIP, but I haven't thought > about the rest yet. Maybe... it's not hard to add uffd-wp for most of the > file-backed memory? Since afaict the kernel handles wr-protect in a quite > straightforward way (do_wp_page() for whatever backend), and uffd-wp can be the > first to trap all of them. I'm not sure whether Andrea has thought about that > or even on how to spread the missing usage to more types of backends (maybe > missing is more special in that it needs to consider page caches). So I'm > copying Andrea too just in case there's further input. Some would be good; we've got requests for it to work on pmem mmaped devices. You do have to be a little careful about semantics though; I'm not sure it's that big of a problem for the wp case, but for the absent case you need to worry about finding an equivalent of madvise or fallocate that cna punch a hole. Dave > Thanks, > > -- > Peter Xu > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK