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

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Message-ID: <b60b783e-a251-f155-3cef-e0fa4a18abd0@redhat.com>
Date:   Mon, 23 Sep 2019 14:20:05 +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 14:07, 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.

ZONE_DEVICE is special as it doesn't have to worry about
onlining/offlining/dumping. Also, it doesn't care about
SECTION_IS_ONLINE, but figuring out how to detect which sub
sections/blocks have a valid memmap is still something to get fixed and
is still getting discussed on MM.

-- 

Thanks,

David / dhildenb