linux-kernel - Re: [PATCH 2/2] mm, memory_hotplug: provide a more generic restrictions for memory hotplug

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Date:   Fri, 5 Apr 2019 12:30:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Oscar Salvador <osalvador@...e.de>, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/2] mm, memory_hotplug: provide a more generic
 restrictions for memory hotplug

On Fri 05-04-19 10:05:09, David Hildenbrand wrote:
> On 05.04.19 09:14, Michal Hocko wrote:
> > On Thu 04-04-19 20:27:41, David Hildenbrand wrote:
> >> On 04.04.19 20:01, Oscar Salvador wrote:
> > [...]
> >>> But I am not really convinced by MHP_SYSTEM_RAM name, and I think we should stick
> >>> with MHP_MEMBLOCK_API because it represents __what__ is that flag about and its
> >>> function, e.g: create memory block devices.
> > 
> > Exactly
> 
> Fine with me for keeping what Oscar has.
> 
> > 
> >> This nicely aligns with the sub-section memory add support discussion.
> >>
> >> MHP_MEMBLOCK_API immediately implies that
> >>
> >> - memory is used as system ram. Memory can be onlined/offlined. Markers
> >>   at sections indicate if the section is online/offline.
> > 
> > No there is no implication like that. It means only that the onlined
> > memory has a sysfs interface. Nothing more, nothing less
> 
> As soon as there is a online/offline interface, you *can* (and user
> space usually *will*) online that memory. Onlining/offlining is only
> defined for memory to be added to the buddy - memory to be used as
> "system ram". Doing it for random device memory will not work / result
> in undefined behavior.

No, not really. We really do not care where the memory comes from. Is it
RAM, NVDIMM, $FOO_BAR_OF_A_FUTURE_BUZZ. We only do care that the memory
can be onlined - user triggered associated with a zone. The memory even
doesn't have to go to the page allocator.
-- 
Michal Hocko
SUSE Labs