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

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Date:   Wed, 26 Jun 2019 10:28:30 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, mhocko@...e.com,
        dan.j.williams@...el.com, pasha.tatashin@...een.com,
        Jonathan.Cameron@...wei.com, anshuman.khandual@....com,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Anshuman Khandual <anshuman.khandual@....com>,
        Rashmica Gupta <rashmica.g@...il.com>
Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory

On 26.06.19 10:15, Oscar Salvador wrote:
> On Wed, Jun 26, 2019 at 10:11:06AM +0200, David Hildenbrand wrote:
>> Back then, I already mentioned that we might have some users that
>> remove_memory() they never added in a granularity it wasn't added. My
>> concerns back then were never fully sorted out.
>>
>> arch/powerpc/platforms/powernv/memtrace.c
>>
>> - Will remove memory in memory block size chunks it never added
>> - What if that memory resides on a DIMM added via MHP_MEMMAP_DEVICE?
>>
>> Will it at least bail out? Or simply break?
>>
>> IOW: I am not yet 100% convinced that MHP_MEMMAP_DEVICE is save to be
>> introduced.
> 
> Uhm, I will take a closer look and see if I can clear your concerns.
> TBH, I did not try to use arch/powerpc/platforms/powernv/memtrace.c
> yet.
> 
> I will get back to you once I tried it out.
> 

BTW, I consider the code in arch/powerpc/platforms/powernv/memtrace.c
very ugly and dangerous. We should never allow to manually
offline/online pages / hack into memory block states.

What I would want to see here is rather:

1. User space offlines the blocks to be used
2. memtrace installs a hotplug notifier and hinders the blocks it wants
to use from getting onlined.
3. memory is not added/removed/onlined/offlined in memtrace code.

CCing the DEVs.

-- 

Thanks,

David / dhildenb