linux-kernel - Re: [PATCH v2 02/10] mm/memory_hotplug: enforce section granularity when onlining/offlining

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Message-ID: <96fd5d6c-a6e3-648b-6d3e-ebfd2d3969d8@redhat.com>
Date:   Tue, 8 Sep 2020 11:14:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baoquan He <bhe@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>
Subject: Re: [PATCH v2 02/10] mm/memory_hotplug: enforce section granularity
 when onlining/offlining

On 25.08.20 04:11, Wei Yang wrote:
> On Mon, Aug 24, 2020 at 12:39:18PM +0200, Oscar Salvador wrote:
>> On Wed, Aug 19, 2020 at 07:59:49PM +0200, David Hildenbrand wrote:
>>> Already two people (including me) tried to offline subsections, because
>>> the function looks like it can deal with it. But we really can only
>>> online/offline full sections that are properly aligned (e.g., we can only
>>> mark full sections online/offline via SECTION_IS_ONLINE).
>>>
>>> Add a simple safety net to document the restriction now. Current users
>>> (core and powernv/memtrace) respect these restrictions.
>>
>> It's been a while since I looked at sub-section handling stuff so sorry to ask
>> this, but was it true that we can hot-{remove,add} sub-section granularity, while
>> we can only online /offline on section granularity?
>>

Yes, we can hot-{remove,add} sub-section granularity ZONE_DEVICE memory,
but not memory to be managed by the buddy.

Examples are
- Memory block devices span 1..X sections and can either be
  online/offline
- We can only mark full sections to be online/offline in sparsemem
- Besides section handling, current onlining/offlining code could only
  work in MAX_ORDER - 1 granularity, not necessarily sub-section
  granularity.

Thanks for having a look.

> 
> Seems you are right.


-- 
Thanks,

David / dhildenb