linux-kernel - Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and anon_always

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Message-ID: <0feef130-59dd-0a31-ed06-e3849a52c75e@redhat.com>
Date:   Thu, 12 Oct 2023 11:31:19 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Ryan Roberts <ryan.roberts@....com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and
 anon_always_mask sysfs files

On 10.10.23 02:20, Andrew Morton wrote:
> On Sun, 08 Oct 2023 09:54:22 +1100 Michael Ellerman <mpe@...erman.id.au> wrote:
> 
>>> I don't know why powerpc's PTE_INDEX_SIZE is variable.
>>
>> To allow a single vmlinux to boot using either the Hashed Page Table
>> MMU, or Radix Tree MMU, which have different page table geometry.
>>
>> That's a pretty crucial feature for distros, so that they can build a
>> single kernel to boot on Power8/9/10.
> 
> Dumb question: why can't distros ship two kernels and have the boot
> loader (or something else) pick the appropriate one?

One answer I keep hearing over and over again is "test matrix 
explosion". So distros only do it when unavoidable: for example, when 
differing PAGE_SIZE is required (e.g., 4k vs 64k) or we're dealing with 
RT support (RT vs !RT).

-- 
Cheers,

David / dhildenb