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Subject: Re: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory
From:   David Hildenbrand <david@redhat.com>
To:     Alex Shi <alex.shi@linux.alibaba.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, virtio-dev@lists.oasis-open.org,
        virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Michael S . Tsirkin" <mst@redhat.com>,
        Sebastien Boeuf <sebastien.boeuf@intel.com>,
        Samuel Ortiz <samuel.ortiz@intel.com>,
        Robert Bradford <robert.bradford@intel.com>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Alexander Potapenko <glider@google.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Anthony Yznaga <anthony.yznaga@oracle.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Young <dyoung@redhat.com>,
        Igor Mammedov <imammedo@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Juergen Gross <jgross@suse.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Len Brown <lenb@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@suse.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Oscar Salvador <osalvador@suse.com>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        Pingfan Liu <kernelfans@gmail.com>, Qian Cai <cai@lca.pw>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Wei Yang <richard.weiyang@gmail.com>
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Message-ID: <e1643897-ebd7-75f8-d271-44f62318aa66@redhat.com>
Date:   Fri, 5 Jun 2020 12:05:40 +0200
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On 05.06.20 11:36, David Hildenbrand wrote:
> On 05.06.20 11:08, David Hildenbrand wrote:
>> On 05.06.20 10:55, Alex Shi wrote:
>>>
>>>
>>> 在 2020/1/9 下午9:48, David Hildenbrand 写道:
>>>> Ping,
>>>>
>>>> I'd love to get some feedback on
>>>>
>>>> a) The remaining MM bits from MM folks (especially, patch #6 and #8).
>>>> b) The general virtio infrastructure (esp. uapi in patch #2) from virtio
>>>> folks.
>>>>
>>>> I'm planning to send a proper v1 (!RFC) once I have all necessary MM
>>>> acks. In the meanwhile, I will do more testing and minor reworks (e.g.,
>>>> fix !CONFIG_NUMA compilation).
>>>
>>>
>>> Hi David,
>>>
>>> Thanks for your work!
>>>
>>> I am trying your https://github.com/davidhildenbrand/linux.git virtio-mem-v5
>>> which works fine for me, but just a 'DMA error' happens when a vm start with
>>> less than 2GB memory, Do I missed sth?
>>
>> Please use the virtio-mem-v4 branch for now, v5 is still under
>> construction (and might be scrapped completely if v4 goes upstream as is).
>>
>> Looks like a DMA issue. Your're hotplugging 1GB, which should not really
>> eat too much memory. There was a similar issue reported by Hui in [1],
>> which boiled down to wrong usage of the swiotlb parameter.
>>
>> In such cases you should always try to reproduce with hotplug of a
>> sam-sized DIMM. E.g., hotplugging a 1GB DIMM should result in the same
>> issue.
>>
>> What does your .config specify for CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE?
>>
>> I'll try to reproduce with v4 briefly.
> 
> I guess I know what's happening here. In case we only have DMA memory
> when booting, we don't reserve swiotlb buffers. Once we hotplug memory
> and online ZONE_NORMAL, we don't have any swiotlb DMA bounce buffers to
> map such PFNs (total 0 (slots), used 0 (slots)).
> 
> Can you try with "swiotlb=force" on the kernel cmdline?

Alternative, looks like you can specify "-m 2G,maxmem=16G,slots=1", to
create proper ACPI tables that indicate hotpluggable memory. (I'll have
to look into QEMU to figure out to always indicate hotpluggable memory
that way).


-- 
Thanks,

David / dhildenb