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Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory
To:     Michal Hocko <mhocko@kernel.org>,
        Rashmica Gupta <rashmica.g@gmail.com>
Cc:     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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Message-ID: <9bcbd574-7e23-5cfe-f633-646a085f935a@redhat.com>
Date:   Thu, 1 Aug 2019 09:18:47 +0200
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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).

-- 

Thanks,

David / dhildenb