linux-kernel - Re: [PATCH v1 0/3] mm/memory_hotplug: Export generic_online

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Message-ID: <8203a264-b327-d512-32ee-054214d2b883@redhat.com>
Date:   Mon, 23 Sep 2019 14:34:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Souptick Joarder <jrdr.linux@...il.com>,
        linux-hyperv@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Oscar Salvador <osalvador@...e.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Qian Cai <cai@....pw>, Sasha Levin <sashal@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Yang <richard.weiyang@...il.com>
Subject: Re: [PATCH v1 0/3] mm/memory_hotplug: Export generic_online_page()

On 23.09.19 14:30, Michal Hocko wrote:
> On Mon 23-09-19 14:20:05, David Hildenbrand wrote:
>> On 23.09.19 14:07, Michal Hocko wrote:
>>> On Mon 23-09-19 13:34:18, David Hildenbrand wrote:
>>>> On 23.09.19 13:15, Michal Hocko wrote:
>>> [...]
>>>>> I am wondering why those pages get onlined when they are, in fact,
>>>>> supposed to be offline.
>>>>>
>>>>
>>>> It's the current way of emulating sub-memory-block hotplug on top of the
>>>> memory bock device API we have. Hyper-V and XEN have been using that for
>>>> a long time.
>>>
>>> Do they really have to use the existing block interface when they in
>>> fact do not operate on the block granularity? Zone device memory already
>>> acts on sub section/block boundaries.
>>>
>>
>> Yes, we need memory blocks, especially for user space to properly online
>> them (as we discussed a while back, to decide on a zone) and for udev
>> events, to e.g., properly reload kexec when memory blocks get
>> added/removed/onlined/offlined.
> 
> Just to make sure I really follow. We need a user interface to control
> where the memory gets onlined but it is the driver which determines
> which part of the block really gets onlined, right?
> 

Yes, it's the drivers job to decide which part of the memory block can
actually get used, and when.

It's a little bit like the driver immediately allocates unbacked memory
again, to free that memory to the buddy once requested. (similar to how
ballooning works).

-- 

Thanks,

David / dhildenb