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Subject: Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid_size
To:     Mike Kravetz <mike.kravetz@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
        linux-s390@vger.kernel.org, sparclinux@vger.kernel.org,
        linux-doc@vger.kernel.org
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>
References: <20200318220634.32100-1-mike.kravetz@oracle.com>
 <20200318220634.32100-2-mike.kravetz@oracle.com>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <ae80063f-cef8-3141-3c6b-9130def5396a@intel.com>
Date:   Thu, 26 Mar 2020 16:10:56 -0700
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Sender: linux-kernel-owner@vger.kernel.org
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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.