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Subject: Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid_size
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From: Dave Hansen <dave.hansen@intel.com>
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Date: Thu, 26 Mar 2020 16:10:56 -0700
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Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Paul Mackerras <paulus@samba.org>, Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>, Heiko Carstens <heiko.carstens@de.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>, Christian Borntraeger <borntraeger@de.ibm.com>, "David S.Miller" <davem@davemloft.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, Jonathan Corbet <corbet@lwn.net>, Longpeng <longpeng2@huawei.com>, Andrew Morton <akpm@linux-foundation.org>, "Shutemov, Kirill" <kirill.shutemov@intel.com>

On 3/26/20 2:56 PM, Mike Kravetz wrote:
> Perhaps it would be best to check hugepages_supported() when parsing
> hugetlb command line options.  If not enabled, throw an error.  This
> will be much easier to do after moving all command line parsing to
> arch independent code.

Yeah, that sounds sane.

> Is that a sufficient way to address this concern?  I think it is a good
> change in any case.

(Thanks to Kirill for pointing this out.)

So, it turns out the x86 huge page enumeration is totally buggered.
X86_FEATURE_PSE is actually meaningless on 64-bit (and 32-bit PAE).  All
CPUs architecturally support 2MB pages regardless of X86_FEATURE_PSE and
the state of CR4.PSE.

So, on x86_64 at least, hugepages_supported() should *always* return 1.

1GB page support can continue to be dependent on X86_FEATURE_GBPAGES.