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Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Rashmica Gupta <rashmica.g@gmail.com>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com,
        anshuman.khandual@arm.com, Vlastimil Babka <vbabka@suse.cz>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <e654aa97-6ab1-4069-60e6-fc099539729e@redhat.com>
Date:   Thu, 1 Aug 2019 09:26:35 +0200
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On 01.08.19 09:24, Michal Hocko wrote:
> On Thu 01-08-19 09:18:47, David Hildenbrand wrote:
>> On 01.08.19 09:17, Michal Hocko wrote:
>>> On Thu 01-08-19 09:06:40, Rashmica Gupta wrote:
>>>> On Wed, 2019-07-31 at 14:08 +0200, Michal Hocko wrote:
>>>>> On Tue 02-07-19 18:52:01, Rashmica Gupta wrote:
>>>>> [...]
>>>>>>> 2) Why it was designed, what is the goal of the interface?
>>>>>>> 3) When it is supposed to be used?
>>>>>>>
>>>>>>>
>>>>>> There is a hardware debugging facility (htm) on some power chips.
>>>>>> To use
>>>>>> this you need a contiguous portion of memory for the output to be
>>>>>> dumped
>>>>>> to - and we obviously don't want this memory to be simultaneously
>>>>>> used by
>>>>>> the kernel.
>>>>>
>>>>> How much memory are we talking about here? Just curious.
>>>>
>>>> From what I've seen a couple of GB per node, so maybe 2-10GB total.
>>>
>>> OK, that is really a lot to keep around unused just in case the
>>> debugging is going to be used.
>>>
>>> I am still not sure the current approach of (ab)using memory hotplug is
>>> ideal. Sure there is some overlap but you shouldn't really need to
>>> offline the required memory range at all. All you need is to isolate the
>>> memory from any existing user and the page allocator. Have you checked
>>> alloc_contig_range?
>>>
>>
>> Rashmica mentioned somewhere in this thread that the virtual mapping
>> must not be in place, otherwise the HW might prefetch some of this
>> memory, leading to errors with memtrace (which checks that in HW).
> 
> Does anything prevent from unmapping the pfn range from the direct
> mapping?

I am not sure about the implications of having
pfn_valid()/pfn_present()/pfn_online() return true but accessing it
results in crashes. (suspend, kdump, whatever other technology touches
online memory)

(sounds more like a hack to me than just going ahead and
removing/readding the memory via a clean interface we have)

-- 

Thanks,

David / dhildenb