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

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Message-ID: <596173b0-310f-7719-d432-11a13e325eab@redhat.com>
Date:   Thu, 6 May 2021 17:38:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Zi Yan <ziy@...dia.com>, Oscar Salvador <osalvador@...e.de>
Cc:     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 06.05.21 17:31, 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.
> 
>>
>> 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.
> 

And just to be clear (I think I mentioned this already to you?): Nack to 
increasing the section size. Nack to increasing the pageblock order. 
Please find different ways to group on gigantic-pages level. There are 
alternative ideas floating around.

Semi-nack to increasing MAX_ORDER. I first want to see 
alloc_contig_range() be able to fully and cleanly handle allocations < 
MAX_ORDER in all cases (especially !CMA and !ZONE_MOVABLE) before we go 
down that path.

-- 
Thanks,

David / dhildenb