linux-kernel - Re: [PATCH 01/13] arm64: mm: Add p?d

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Date:   Mon, 18 Feb 2019 14:11:40 +0000
From:   Steven Price <steven.price@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-mm@...ck.org, Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/13] arm64: mm: Add p?d_large() definitions

On 18/02/2019 11:29, Peter Zijlstra wrote:
> On Fri, Feb 15, 2019 at 05:02:22PM +0000, Steven Price wrote:
> 
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index de70c1eabf33..09d308921625 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -428,6 +428,7 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>>  				 PMD_TYPE_TABLE)
>>  #define pmd_sect(pmd)		((pmd_val(pmd) & PMD_TYPE_MASK) == \
>>  				 PMD_TYPE_SECT)
>> +#define pmd_large(x)		pmd_sect(x)
>>  
>>  #if defined(CONFIG_ARM64_64K_PAGES) || CONFIG_PGTABLE_LEVELS < 3
>>  #define pud_sect(pud)		(0)
>> @@ -435,6 +436,7 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>>  #else
>>  #define pud_sect(pud)		((pud_val(pud) & PUD_TYPE_MASK) == \
>>  				 PUD_TYPE_SECT)
>> +#define pud_large(x)		pud_sect(x)
>>  #define pud_table(pud)		((pud_val(pud) & PUD_TYPE_MASK) == \
>>  				 PUD_TYPE_TABLE)
>>  #endif
> 
> So on x86 p*d_large() also matches p*d_huge() and thp, But it is not
> clear to me this p*d_sect() thing does so, given your definitions.
> 
> See here why I care:
> 
>   http://lkml.kernel.org/r/20190201124741.GE31552@hirez.programming.kicks-ass.net
> 

pmd_huge()/pud_huge() unfortunately are currently defined as '0' if
!CONFIG_HUGETLB_PAGE and for this reason I was avoiding using them.
While most code would reasonably not care about huge pages in that build
configuration, the likes of the debugfs page table dump code needs to be
able to recognise them in all build configurations. I believe the
situation is the same on arm64 and x86.

The other quirk is that higher levels are not supported for HUGETLB on
some arch implementations. For example arm64 provides no definition for
pgd_huge() so falls back to the generic defined as '0' implementation.
The only architecture I can see that defines this is powerpc. Keeping
this as '0' ensures that the otherwise dead code in other places is
compiled out.

Where p?d_huge is defined by the arch code the intention is that
p?d_large(x)==p?d_huge(x).

Thanks,

Steve