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

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Date:   Mon, 18 Feb 2019 17:22:32 +0000
From:   Steven Price <steven.price@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        James Morse <james.morse@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 01/13] arm64: mm: Add p?d_large() definitions

On 18/02/2019 17:04, Mark Rutland wrote:
> On Mon, Feb 18, 2019 at 03:30:38PM +0000, Steven Price wrote:
>> On 18/02/2019 15:06, Peter Zijlstra wrote:
>>> On Mon, Feb 18, 2019 at 02:29:52PM +0000, Mark Rutland wrote:
>>>> I think that Peter means p?d_huge(x) should imply p?d_large(x), e.g.
>>>>
>>>> #define pmd_large(x) \
>>>> 	(pmd_sect(x) || pmd_huge(x) || pmd_trans_huge(x))
>>>>
>>>> ... which should work regardless of CONFIG_HUGETLB_PAGE.
>>>
>>> Yep, that.
>>
>> I'm not aware of a situation where pmd_huge(x) is true but pmd_sect(x)
>> isn't. Equally for pmd_huge(x) and pmd_trans_huge(x).
>>
>> What am I missing?
> 
> Having dug for a bit, I think you're right in asserting that pmd_sect()
> should cover those.
> 
> I had worried that wouldn't cater for contiguous pmd entries, but those
> have to be contiguous section entries, so they get picked up.
> 
> That said, do we have any special handling for contiguous PTEs? We use
> those in kernel mappings regardless of hugetlb support, and I didn't
> spot a pte_large() helper.

There's no special handling for contiguous PTEs because the page walk
code doesn't care - each PTE is valid individually even if it is part of
a contiguous group. So the walker can descend all levels in this case.
pte_large() if it existed would therefore always return 0.

The pte_entry() callback obviously might go looking for the contiguous
bit so that it can annotate the output correctly but that's different
from a 'large' page. The code in arch/arm64/mm/dump.c simply looks for
the PTE_CONT bit being set to do this annotation.

Steve