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Date:   Thu, 12 Oct 2023 11:31:19 +0200
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Subject: Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and
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To:     Andrew Morton <akpm@linux-foundation.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Ryan Roberts <ryan.roberts@arm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Matthew Wilcox <willy@infradead.org>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yu Zhao <yuzhao@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Itaru Kitayama <itaru.kitayama@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org
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 <87fs2mrqld.fsf@mail.lhotse>
 <20231009172029.e558c08dd90d9b94fec3a036@linux-foundation.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 10.10.23 02:20, Andrew Morton wrote:
> On Sun, 08 Oct 2023 09:54:22 +1100 Michael Ellerman <mpe@ellerman.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