Re: [PATCH v3 03/34] arm: mm: Add p?d_large() definitions

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Steven Price <steven.price@arm.com>
Cc: "Mark Rutland" <Mark.Rutland@arm.com>,
	x86@kernel.org, "Arnd Bergmann" <arnd@arndb.de>,
	"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Will Deacon" <will.deacon@arm.com>,
	linux-kernel@vger.kernel.org,
	"Russell King" <linux@armlinux.org.uk>,
	linux-mm@kvack.org, "Jérôme Glisse" <jglisse@redhat.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Andy Lutomirski" <luto@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"James Morse" <james.morse@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	linux-arm-kernel@lists.infradead.org, "Liang,
	Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v3 03/34] arm: mm: Add p?d_large() definitions
Date: Mon, 4 Mar 2019 16:10:54 +0300	[thread overview]
Message-ID: <20190304131054.goxpqsosolkg7khf@kshutemo-mobl1> (raw)
In-Reply-To: <974310a0-0114-9a0c-9041-4e0394c4b9aa@arm.com>

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