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From: Steven Price <steven.price@arm.com>
To: Dave Hansen <dave.hansen@intel.com>, linux-mm@kvack.org
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,
	"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 27/34] mm: pagewalk: Add 'depth' parameter to pte_hole
Date: Thu, 28 Feb 2019 11:28:01 +0000	[thread overview]
Message-ID: <02b9ec67-75c5-4a36-9110-cc4ba6ee4f94@arm.com> (raw)
In-Reply-To: <aece3046-6040-e2ec-fcd7-204113d40eb7@intel.com>

On 27/02/2019 17:38, Dave Hansen wrote:
> On 2/27/19 9:06 AM, Steven Price wrote:
>>  #ifdef CONFIG_SHMEM
>>  static int smaps_pte_hole(unsigned long addr, unsigned long end,
>> -		struct mm_walk *walk)
>> +			  __always_unused int depth, struct mm_walk *walk)
>>  {
> 
> I think this 'depth' argument is a mistake.  It's synthetic and it's
> surely going to be a source of bugs.
> 
> The page table dumpers seem to be using this to dump out the "name" of a
> hole which seems a bit bogus in the first place.  I'd much rather teach
> the dumpers about the length of the hole, "the hole is 0x12340000 bytes
> long", rather than "there's a hole at this level".

I originally started by trying to calculate the 'depth' from (end -
addr), e.g. for arm64:

level = 4 - (ilog2(end - addr) - PAGE_SHIFT) / (PAGE_SHIFT - 3)

However there are two issues that I encountered:

* walk_page_range() takes a range of addresses to walk. This means that
holes at the beginning/end of the range are clamped to the address
range. This particularly shows up at the end of the range as I use ~0ULL
as the end which leads to (~0ULL - addr) which is 1 byte short of the
desired size. Obviously that particular corner-case is easy to work
round, but it seemed fragile.

* The above definition for arm64 isn't correct in all cases. You need to
account for things like CONFIG_PGTABLE_LEVELS. Other architectures also
have various quirks in their page tables.

I guess I could try something like:

static int get_level(unsigned long addr, unsigned long end)
{
	/* Add 1 to account for ~0ULL */
	unsigned long size = (end - addr) + 1;
	if (size < PMD_SIZE)
		return 4;
	else if (size < PUD_SIZE)
		return 3;
	else if (size < P4D_SIZE)
		return 2;
	else if (size < PGD_SIZE)
		return 1;
	return 0;
}

There are two immediate problems with that:

 * The "+1" to deal with ~0ULL is fragile

 * PGD_SIZE isn't what you might expect, it's not defined for most
architectures and arm64/x86 use it as the size of the PGD table.
Although that's easy enough to fix up.

Do you think a function like above would be preferable?

The other option would of course be to just drop the information from
the debugfs file about at which level the holes are. But it can be
useful information to see whether there are empty levels in the page
table structure. Although this is an area where x86 and arm64 differ
currently (x86 explicitly shows the gaps, arm64 doesn't), so if x86
doesn't mind losing that functionality that would certainly simplify things!

Thanks,

Steve

  reply	other threads:[~2019-02-28 11:28 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-27 17:05 [PATCH v3 00/34] Convert x86 & arm64 to use generic page walk Steven Price
2019-02-27 17:05 ` [PATCH v3 01/34] alpha: mm: Add p?d_large() definitions Steven Price
2019-02-27 17:05 ` [PATCH v3 02/34] arc: " Steven Price
2019-02-27 18:18   ` Vineet Gupta
2019-02-27 17:05 ` [PATCH v3 03/34] arm: " Steven Price
2019-03-01 21:47   ` Kirill A. Shutemov
2019-03-04 11:56     ` Steven Price
2019-03-04 13:10       ` Kirill A. Shutemov
2019-02-27 17:05 ` [PATCH v3 04/34] arm64: " Steven Price
2019-02-27 17:05 ` [PATCH v3 05/34] c6x: " Steven Price
2019-03-01 21:48   ` Kirill A. Shutemov
2019-03-04 12:01     ` Steven Price
2019-03-04 13:11       ` Kirill A. Shutemov
2019-02-27 17:05 ` [PATCH v3 06/34] csky: " Steven Price
2019-03-01 21:51   ` Kirill A. Shutemov
2019-02-27 17:05 ` [PATCH v3 07/34] hexagon: " Steven Price
2019-02-27 17:05 ` [PATCH v3 08/34] ia64: " Steven Price
2019-03-01 21:57   ` Kirill A. Shutemov
2019-03-04 13:16     ` Steven Price
2019-03-04 19:06       ` Luck, Tony
2019-03-06 13:45         ` Steven Price
2019-02-27 17:05 ` [PATCH v3 09/34] m68k: " Steven Price
2019-02-27 19:27   ` Geert Uytterhoeven
2019-02-28 11:36     ` Mike Rapoport
2019-02-28 11:53       ` Geert Uytterhoeven
2019-02-28 12:04         ` Steven Price
2019-03-01 11:45           ` Mike Rapoport
2019-02-27 17:05 ` [PATCH v3 10/34] microblaze: " Steven Price
2019-02-27 17:05 ` [PATCH v3 11/34] mips: " Steven Price
2019-02-28  2:15   ` Paul Burton
2019-02-28 12:11     ` Steven Price
2019-02-28 18:55       ` Paul Burton
2019-03-01 11:02         ` Steven Price
2019-02-27 17:05 ` [PATCH v3 12/34] nds32: " Steven Price
2019-02-27 17:05 ` [PATCH v3 13/34] nios2: " Steven Price
2019-02-27 17:05 ` [PATCH v3 14/34] openrisc: " Steven Price
2019-02-27 17:05 ` [PATCH v3 15/34] parisc: " Steven Price
2019-02-27 18:54   ` Helge Deller
2019-03-01 22:12     ` Kirill A. Shutemov
2019-03-05 21:45       ` Helge Deller
2019-02-27 17:05 ` [PATCH v3 16/34] powerpc: " Steven Price
2019-02-27 17:05 ` [PATCH v3 17/34] riscv: " Steven Price
2019-02-27 17:05 ` [PATCH v3 18/34] s390: " Steven Price
2019-02-27 17:40   ` Martin Schwidefsky
2019-02-28 11:43     ` Steven Price
2019-02-27 17:05 ` [PATCH v3 19/34] sh: " Steven Price
2019-02-27 17:05 ` [PATCH v3 20/34] sparc: " Steven Price
2019-02-27 18:38   ` David Miller
2019-02-28 11:49     ` Steven Price
2019-02-27 17:05 ` [PATCH v3 21/34] um: " Steven Price
2019-02-27 17:05 ` [PATCH v3 22/34] unicore32: " Steven Price
2019-02-27 17:05 ` [PATCH v3 23/34] xtensa: " Steven Price
2019-02-27 17:29   ` Max Filippov
2019-02-27 17:05 ` [PATCH v3 24/34] mm: Add generic p?d_large() macros Steven Price
2019-02-27 17:05 ` [PATCH v3 25/34] mm: pagewalk: Add p4d_entry() and pgd_entry() Steven Price
2019-02-27 17:06 ` [PATCH v3 26/34] mm: pagewalk: Allow walking without vma Steven Price
2019-02-27 17:06 ` [PATCH v3 27/34] mm: pagewalk: Add 'depth' parameter to pte_hole Steven Price
2019-02-27 17:38   ` Dave Hansen
2019-02-28 11:28     ` Steven Price [this message]
2019-02-28 19:00       ` Dave Hansen
2019-03-01 11:24         ` Steven Price
2019-02-27 17:06 ` [PATCH v3 28/34] mm: pagewalk: Add test_p?d callbacks Steven Price
2019-02-27 17:06 ` [PATCH v3 29/34] arm64: mm: Convert mm/dump.c to use walk_page_range() Steven Price
2019-02-27 17:06 ` [PATCH v3 30/34] x86/mm: Point to struct seq_file from struct pg_state Steven Price
2019-02-27 17:06 ` [PATCH v3 31/34] x86/mm+efi: Convert ptdump_walk_pgd_level() to take a mm_struct Steven Price
2019-02-27 17:06 ` [PATCH v3 32/34] x86/mm: Convert ptdump_walk_pgd_level_debugfs() to take an mm_struct Steven Price
2019-02-27 17:06 ` [PATCH v3 33/34] x86/mm: Convert ptdump_walk_pgd_level_core() " Steven Price
2019-02-27 17:06 ` [PATCH v3 34/34] x86: mm: Convert dump_pagetables to use walk_page_range Steven Price

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