linux-kernel - Re: [PATCH v3 03/34] arm: mm: Add p?d

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Date:   Mon, 4 Mar 2019 11:56:13 +0000
From:   Steven Price <steven.price@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Mark Rutland <Mark.Rutland@....com>, 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, Russell King <linux@...linux.org.uk>,
        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,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v3 03/34] arm: mm: Add p?d_large() definitions

On 01/03/2019 21:47, Kirill A. Shutemov wrote:
> On Wed, Feb 27, 2019 at 05:05:37PM +0000, Steven Price wrote:
>> walk_page_range() is going to be allowed to walk page tables other than
>> those of user space. For this it needs to know when it has reached a
>> 'leaf' entry in the page tables. This information will be provided by the
>> p?d_large() functions/macros.
>>
>> For arm, we already provide most p?d_large() macros. Add a stub for PUD
>> as we don't have huge pages at that level.
> 
> We do not have PUD for 2- and 3-level paging. Macros from generic header
> should cover it, shouldn't it?
> 

I'm not sure of the reasoning behind this, but levels are folded in a
slightly strange way. arm/include/asm/pgtable.h defines
__ARCH_USE_5LEVEL_HACK which means:

PGD has 2048 (2-level) or 4 (3-level) entries which are always
considered 'present' (pgd_present() returns 1 defined in
asm-generic/pgtables-nop4d-hack.h).

P4D has 1 entry which is always present (see asm-generic/5level-fixup.h)

PUD has 1 entry (see asm-generic/pgtable-nop4d-hack.h). This is always
present for 2-level, and present only if the first level of real page
table is present with a 3-level.

PMD/PTE are as you might expect.

So in terms of tables which are more than one entry you have PGD,
(optionally) PMD, PTE. But the levels which actually read the table
entries are PUD, PMD, PTE.

This means that the corresponding p?d_large() macros are needed for
PUD/PMD as that is where the actual entries are read. The asm-generic
files provide the definitions for PGD/P4D.

Steve