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From:   "Zi Yan" <ziy@nvidia.com>
To:     "David Hildenbrand" <david@redhat.com>
Cc:     "Oscar Salvador" <osalvador@suse.de>,
        "Michael Ellerman" <mpe@ellerman.id.au>,
        "Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
        "Thomas Gleixner" <tglx@linutronix.de>, x86@kernel.org,
        "Andy Lutomirski" <luto@kernel.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Mike Rapoport" <rppt@kernel.org>,
        "Anshuman Khandual" <anshuman.khandual@arm.com>,
        "Michal Hocko" <mhocko@suse.com>,
        "Dan Williams" <dan.j.williams@intel.com>,
        "Wei Yang" <richard.weiyang@linux.alibaba.com>,
        linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH 0/7] Memory hotplug/hotremove at subsection size
Date:   Thu, 06 May 2021 11:37:15 -0400
Message-ID: <9D7FD316-988E-4B11-AC1C-64FF790BA79E@nvidia.com>
In-Reply-To: <fb60eabd-f8ef-2cb1-7338-7725efe3c286@redhat.com>
References: <20210506152623.178731-1-zi.yan@sent.com>
 <fb60eabd-f8ef-2cb1-7338-7725efe3c286@redhat.com>
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On 6 May 2021, at 11:31, David Hildenbrand wrote:

> On 06.05.21 17:26, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> Hi all,
>>
>> This patchset tries to remove the restriction on memory hotplug/hotrem=
ove
>> 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 i=
ndependent
>> of memory section size, as small as 2MB (the subsection size).
>
> ... which doesn't make any sense as we can only online/offline whole me=
mory block devices.

Why limit the memory block size to section size? Patch 3 removes the rest=
riction
by using (start_pfn, nr_pages) to allow memory block size goes below sect=
ion size.
Also we have subsection bitmap available to tell us which subsection is o=
nline,
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 pag=
eblock
>> size without increasing memory hotplug/hotremove granularity at the sa=
me time,
>
> Gah, no. Please no. No.
>
>> so that the kernel can allocator 1GB pages using buddy allocator and u=
tilizes
>> existing pageblock based anti-fragmentation, paving the road for 1GB T=
HP
>> support[2].
>
> Not like this, please no.
>
>>
>> The patchset utilizes the existing subsection support[3] and changes t=
he
>> section size alignment checks to subsection size alignment checks. The=
re are
>> also changes to pageblock code to support partial pageblocks, when pag=
eblock
>> size is increased along with MAX_ORDER. Increasing pageblock size can =
enable
>> kernel to utilize existing anti-fragmentation mechanism for gigantic p=
age
>> allocations.
>
> Please not like this.
>
>>
>> The last patch increases SECTION_SIZE_BITS to demonstrate the use of m=
emory
>> 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 yo=
ur
>> feedback.
>
> Please not like this.

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

Thanks.


=E2=80=94
Best Regards,
Yan Zi

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