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

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Date:   Mon, 4 Mar 2019 16:10:54 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Steven Price <steven.price@....com>
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 Mon, Mar 04, 2019 at 11:56:13AM +0000, Steven Price wrote:
> 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.

Makes sense.

Only additional thing worth nothing that ARM in 2-level paging case folds
PMD manually without help from generic headres.

I'm partly responsible for the mess with folding. Sorry that you need to
explain this to :P

-- 
 Kirill A. Shutemov