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

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Date:   Wed, 18 Mar 2020 15:52:46 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Dave Hansen <dave.hansen@...el.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

On 3/18/20 3:15 PM, Dave Hansen wrote:
> 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.

Sounds good.  I'll incorporate those changes into a v2, unless someone
else with has a different opinion.

BTW, this patch should not really change the way the code works today.
It is mostly a movement of code.  Unless I am missing something, the
existing code will always allow setup of PMD_SIZE hugetlb pages.
-- 
Mike Kravetz