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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Andrew Morton <akpm@linux-foundation.org>
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>,
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        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>,
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        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
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        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v6 4/9] mm: thp: Introduce anon_orders and
 anon_always_mask sysfs files
In-Reply-To: <20231009172029.e558c08dd90d9b94fec3a036@linux-foundation.org>
References: <20230929114421.3761121-1-ryan.roberts@arm.com>
 <20230929114421.3761121-5-ryan.roberts@arm.com>
 <20230929155530.a51e68e03e47a06b6b84c689@linux-foundation.org>
 <87fs2mrqld.fsf@mail.lhotse>
 <20231009172029.e558c08dd90d9b94fec3a036@linux-foundation.org>
Date:   Thu, 12 Oct 2023 22:07:26 +1100
Message-ID: <87o7h4qett.fsf@mail.lhotse>
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Andrew Morton <akpm@linux-foundation.org> writes:
> 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?

I'm not a grub expert, but AFAIK it doesn't support loading a different
kernel based on CPU/firwmare features. I'm quite sure it can't do that
on powerpc at least.

We also have another bootloader (petitboot) that is still supported by
some distros, and can't do that.

The other problem is like David says, distros are generally reluctant to
add new kernel configurations unless they absolutely have to. It adds
more work for them, more things to track, and can confuse users leading
to spurious bug reports.

cheers