linux-kernel - Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

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Message-ID: <5e6137c9-5269-5756-beaa-d116652be8b9@redhat.com>
Date:   Thu, 1 Aug 2019 09:31:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Rashmica Gupta <rashmica.g@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        pasha.tatashin@...een.com, Jonathan.Cameron@...wei.com,
        anshuman.khandual@....com, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On 01.08.19 09:26, David Hildenbrand wrote:
> 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)

(oneidea: we could of course go ahead and mark the pages PG_offline
before unmapping the pfn range to work around these issues)

> 
> (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