Date:   Wed, 18 Jul 2018 11:56:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Balbir Singh <bsingharora@...il.com>,
        Baoquan He <bhe@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Young <dyoung@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Huang Ying <ying.huang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>, Jan Kara <jack@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Juergen Gross <jgross@...e.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Mel Gorman <mgorman@...e.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Miles Chen <miles.chen@...iatek.com>,
        Oscar Salvador <osalvador@...hadventures.net>,
        Paul Mackerras <paulus@...ba.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Rashmica Gupta <rashmica.g@...il.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v1 00/10] mm: online/offline 4MB chunks controlled by
 device driver

On 16.07.2018 22:05, Michal Hocko wrote:
> On Mon 16-07-18 21:48:59, David Hildenbrand wrote:
>> On 11.06.2018 14:33, David Hildenbrand wrote:
>>> On 11.06.2018 13:56, Michal Hocko wrote:
>>>> On Mon 11-06-18 13:53:49, David Hildenbrand wrote:
>>>>> On 24.05.2018 23:07, David Hildenbrand wrote:
>>>>>> On 24.05.2018 16:22, Michal Hocko wrote:
>>>>>>> I will go over the rest of the email later I just wanted to make this
>>>>>>> point clear because I suspect we are talking past each other.
>>>>>>
>>>>>> It sounds like we are now talking about how to solve the problem. I like
>>>>>> that :)
>>>>>>
>>>>>
>>>>> Hi Michal,
>>>>>
>>>>> did you have time to think about the details of your proposed idea?
>>>>
>>>> Not really. Sorry about that. It's been busy time. I am planning to
>>>> revisit after merge window closes.
>>>>
>>>
>>> Sure no worries, I still have a bunch of other things to work on. But it
>>> would be nice to clarify soon in which direction I have to head to get
>>> this implemented and upstream (e.g. what I proposed, what you proposed
>>> or maybe something different).
>>>
>> I would really like to make progress here.
>>
>> I pointed out basic problems/questions with the proposed alternative. I
>> think I answered all your questions. But you also said that you are not
>> going to accept the current approach. So some decision has to be made.
>>
>> Although it's very demotivating and frustrating (I hope not all work in
>> the MM area will be like this), if there is no guidance on how to
>> proceed, I'll have to switch to adding/removing/onlining/offlining whole
>> segments. This is not what I want, but maybe this has a higher chance of
>> getting reviews/acks.
>>
>> Understanding that you are busy, please if you make suggestions, follow
>> up on responses.
> 
> I plan to get back to this. It's busy time with too many things
> happening both upstream and on my work table as well. Sorry about that.
> I do understand your frustration but there is only that much time I
> have. There are not that many people to review this code unfortunately.
> 
> In principle though, I still maintain my position that the memory
> hotplug code is way too subtle to add more on top. Maybe the code can be
> reworked to be less section oriented but that will be a lot of work.
> If you _really_ need a smaller granularity I do not have a better
> suggestion than to emulate that on top of sections. I still have to go
> back to your last emails though.
> 

The only way I see doing the stuff on top will be using a new bit for
marking pages as offline (PageOffline - Patch 1).

When a section is added, all pages are initialized to PageOffline.

online_pages() can be then hindered to online specific pages using the
well known hook set_online_page_callback().

In my driver, I can manually "soft offline" parts, setting them to
PageOffline or "soft online" them again (including clearing PageOffline).

offline_pages() can then skip all pages that are already "soft offline"
- PageOffline set - and effectively set the section offline.


Without this new bit offline_pages() cannot know if a page is actually
offline or simply reserved by some other part of the system. Imagine
that all parts of a section are "soft offline". Now I want to offline
the section and remove the memory. I would have to temporarily online
all pages again, adding them to the buddy in order to properly offline
them using offline_pages(). Prone to races as these pages must not be
touched.

So in summary, PageOffline would have to be used but
online_pages/offline_pages would continue calling e.g. notifiers on
segment basis. Boils down to patch 1 and another patch that skips pages
that are already offline in offline_pages().

Once you have some spare cycles, please let me know what you think or
which other alternatives you see. Thanks.

-- 

Thanks,

David / dhildenb

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