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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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Organization: Red Hat GmbH
Message-ID: <6b4724bf-84b5-9880-5464-1908425d106d@redhat.com>
Date:   Fri, 5 Jun 2020 11:36:09 +0200
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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?

-- 
Thanks,

David / dhildenb