linux-kernel - Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

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Message-ID: <9D7FD316-988E-4B11-AC1C-64FF790BA79E@nvidia.com>
Date:   Thu, 06 May 2021 11:37:15 -0400
From:   "Zi Yan" <ziy@...dia.com>
To:     "David Hildenbrand" <david@...hat.com>
Cc:     "Oscar Salvador" <osalvador@...e.de>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
        "Thomas Gleixner" <tglx@...utronix.de>, x86@...nel.org,
        "Andy Lutomirski" <luto@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mike Rapoport" <rppt@...nel.org>,
        "Anshuman Khandual" <anshuman.khandual@....com>,
        "Michal Hocko" <mhocko@...e.com>,
        "Dan Williams" <dan.j.williams@...el.com>,
        "Wei Yang" <richard.weiyang@...ux.alibaba.com>,
        linux-ia64@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size

On 6 May 2021, at 11:31, David Hildenbrand wrote:

> On 06.05.21 17:26, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> Hi all,
>>
>> This patchset tries to remove the restriction on memory hotplug/hotremove
>> granularity, which is always greater or equal to memory section size[1].
>> With the patchset, kernel is able to online/offline memory at a size independent
>> of memory section size, as small as 2MB (the subsection size).
>
> ... which doesn't make any sense as we can only online/offline whole memory block devices.

Why limit the memory block size to section size? Patch 3 removes the restriction
by using (start_pfn, nr_pages) to allow memory block size goes below section size.
Also we have subsection bitmap available to tell us which subsection is online,
there is no reason to force memory block size to match section size.

>
>>
>> The motivation is to increase MAX_ORDER of the buddy allocator and pageblock
>> size without increasing memory hotplug/hotremove granularity at the same time,
>
> Gah, no. Please no. No.
>
>> so that the kernel can allocator 1GB pages using buddy allocator and utilizes
>> existing pageblock based anti-fragmentation, paving the road for 1GB THP
>> support[2].
>
> Not like this, please no.
>
>>
>> The patchset utilizes the existing subsection support[3] and changes the
>> section size alignment checks to subsection size alignment checks. There are
>> also changes to pageblock code to support partial pageblocks, when pageblock
>> size is increased along with MAX_ORDER. Increasing pageblock size can enable
>> kernel to utilize existing anti-fragmentation mechanism for gigantic page
>> allocations.
>
> Please not like this.
>
>>
>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of memory
>> hotplug/hotremove subsection, but is not intended to be merged as is. It is
>> there in case one wants to try this out and will be removed during the final
>> submission.
>>
>> Feel free to give suggestions and comments. I am looking forward to your
>> feedback.
>
> Please not like this.

Do you mind sharing more useful feedback instead of just saying a lot of No?

Thanks.


—
Best Regards,
Yan Zi

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