linux-kernel - Re: [RFC PATCH 1/7] mm: sparse: set/clear subsection bitmap when pages are onlined/offlined.

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Date:   Thu, 6 May 2021 21:14:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Zi Yan <ziy@...dia.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 1/7] mm: sparse: set/clear subsection bitmap when
 pages are onlined/offlined.

>> But glimpsing at patch #2, I'd rather stop right away digging deeper into this series :)
> 
> What is the issue of patch 2, which makes pageblock_order a variable all the time? BTW, patch 2 fixes a bug by exporting pageblock_order, since when HUGETLB_PAGE_SIZE_VARIABLE is set, virtio-mem will not see pageblock_order as a variable, which could happen for PPC_BOOK2S_64 with virtio-men enabled, right? Or is this an invalid combination?

virtio_mem is x86_64 only. aarch64 and s390x prototypes are available.

If I understood "Make pageblock_order a variable and
set it to the max of HUGETLB_PAGE_ORDER, MAX_ORDER - 1" correctly, 
you're setting the pageblock_order on x86_64 to 4M. That mean's you're 
no longer grouping for THP but MAX_ORDER - 1, which is not what we want. 
We want to optimize for THP.

Also, that would affect virtio-balloon with free page reporting (report 
only 4 MiB chunks not 2 MiB chunks).

> 
>>
>> I think what would really help is drafting a design of how it all could look like and then first discussing the high-level design, investigating how it could play along with all existing users, existing workloads, and existing use cases. Proposing such changes without a clear picture in mind and a high-level overview might give you some unpleasant reactions from some of the developers around here ;)
> 
> Please see my other email for a high-level design. Also I sent the patchset as a RFC to gather information on users, workloads, use cases I did not know about and I learnt a lot from your replies. :) Feedback is always welcome, but I am not sure why it needs to make people unpleasant. ;)

Rather the replies might be unpleasant ;)

-- 
Thanks,

David / dhildenb