linux-kernel - Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid

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Message-ID: <831a0773-1ba6-4d72-44b9-7472123b8528@intel.com>
Date:   Wed, 18 Mar 2020 15:15:43 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "David S . Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Longpeng <longpeng2@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] hugetlbfs: add arch_hugetlb_valid_size

Hi Mike,

The series looks like a great idea to me.  One nit on the x86 bits,
though...

> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index 5bfd5aef5378..51e6208fdeec 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -181,16 +181,25 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  #endif /* CONFIG_HUGETLB_PAGE */
>  
>  #ifdef CONFIG_X86_64
> +bool __init arch_hugetlb_valid_size(unsigned long long size)
> +{
> +	if (size == PMD_SIZE)
> +		return true;
> +	else if (size == PUD_SIZE && boot_cpu_has(X86_FEATURE_GBPAGES))
> +		return true;
> +	else
> +		return false;
> +}

I'm pretty sure it's possible to have a system without 2M/PMD page
support.  We even have a handy-dandy comment about it in
arch/x86/include/asm/required-features.h:

	#ifdef CONFIG_X86_64
	#ifdef CONFIG_PARAVIRT
	/* Paravirtualized systems may not have PSE or PGE available */
	#define NEED_PSE        0
	...

I *think* you need an X86_FEATURE_PSE check here to be totally correct.

	if (size == PMD_SIZE && cpu_feature_enabled(X86_FEATURE_PSE))
		return true;

BTW, I prefer cpu_feature_enabled() to boot_cpu_has() because it
includes disabled-features checking.  I don't think any of it matters
for these specific features, but I generally prefer it on principle.