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

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Date:   Mon, 18 Feb 2019 13:45:07 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Price <steven.price@....com>, 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 Mon, Feb 18, 2019 at 12:29:22PM +0100, 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

I believe it does not.

IIUC our p?d_huge() helpers implicitly handle contiguous entries. That's
where you have $N entries in the current level of table that the TLB can
cache together as one.

Our p?d_sect() helpers only match section entries. That's where we map
an entire next-level-table's worth of VA space with a single entry at
the current level.

Thanks,
Mark.