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From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Will Deacon <will@kernel.org>
Cc: "Steven Price" <steven.price@arm.com>,
	linux-mm@kvack.org, "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>,
	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>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org, "Liang,
	Kan" <kan.liang@linux.intel.com>
Subject: Re: [PATCH v9 00/21] Generic page walk and ptdump
Date: Fri, 26 Jul 2019 11:33:14 +0530
Message-ID: <40adc5ea-1125-d821-267d-3621732775d6@arm.com> (raw)
In-Reply-To: <20190725093036.dzn6uulcihhkohm2@willie-the-truck>



On 07/25/2019 03:00 PM, Will Deacon wrote:
> On Thu, Jul 25, 2019 at 02:39:22PM +0530, Anshuman Khandual wrote:
>> On 07/24/2019 07:05 PM, Steven Price wrote:
>>> There isn't any problem as such with using p?d_large macros. However the
>>> name "large" has caused confusion in the past. In particular there are
>>> two types of "large" page:
>>>
>>> 1. leaf entries at high levels than normal ('sections' on Arm, for 4K
>>> pages this gives you 2MB and 1GB pages).
>>>
>>> 2. sets of contiguous entries that can share a TLB entry (the
>>> 'Contiguous bit' on Arm - which for 4K pages gives you 16 entries = 64
>>> KB 'pages').
>>
>> This is arm64 specific and AFAIK there are no other architectures where there
>> will be any confusion wrt p?d_large() not meaning a single entry.
>>
>> As you have noted before if we are printing individual entries with PTE_CONT
>> then they need not be identified as p??d_large(). In which case p?d_large()
>> can just safely point to p?d_sect() identifying regular huge leaf entries.
> 
> Steven's stuck in the middle of things here, but I do object to p?d_large()
> because I find it bonkers to have p?d_large() and p?d_huge() mean completely
> different things when they are synonyms in the English language.

Agreed that both p?d_large() and p?d_huge() should not exist at the same time
when they imply the same thing. I believe all these name proliferation happened
because THP, HugeTLB and kernel large mappings like linear, vmemmap, ioremap etc
which the platform code had to deal with in various forms.

> 
> Yes, p?d_leaf() matches the terminology used by the Arm architecture, but
> given that most page table structures are arranged as a 'tree', then it's
> not completely unreasonable, in my opinion. If you have a more descriptive
> name, we could use that instead. We could also paint it blue.

The alternate name chosen p?d_leaf() is absolutely fine and it describes the
entry as intended. There is no disagreement over that. My original concern
was introduction of yet another page table helper.

> 
>>> In many cases both give the same effect (reduce pressure on TLBs and
>>> requires contiguous and aligned physical addresses). But for this case
>>> we only care about the 'leaf' case (because the contiguous bit makes no
>>> difference to walking the page tables).
>>
>> Right and we can just safely identify section entries with it. What will be
>> the problem with that ? Again this is only arm64 specific.
>>
>>>
>>> As far as I'm aware p?d_large() currently implements the first and
>>> p?d_(trans_)huge() implements either 1 or 2 depending on the architecture.
>>
>> AFAIK option 2 exists only on arm6 platform. IIUC generic MM requires two
>> different huge page dentition from platform. HugeTLB identifies large entries
>> at PGD|PUD|PMD after converting it's content into PTE first. So there is no
>> need for direct large page definitions for other levels.
>>
>> 1. THP		- pmd_trans_huge()
>> 2. HugeTLB	- pte_huge()	   CONFIG_ARCH_WANT_GENERAL_HUGETLB is set
>>
>> A simple check for p?d_large() on mm/ and include/linux shows that there are
>> no existing usage for these in generic MM. Hence it is available.
> 
> Alternatively, it means we have a good opportunity to give it a better name
> before it spreads into the core code.

Fair enough, that is another way. So you expect existing platform definitions
for p?d_large()/p?d_huge() getting cleaned up and to start using new p?d_leaf()
instead ?

> 
>> IMHO the new addition of p?d_leaf() can be avoided and p?d_large() should be
>> cleaned up (if required) in platforms and used in generic functions.
> 
> Again, I disagree and think p?d_large() should be confined to arch code
> if it sticks around at all.

All of those instances should migrate to using p?d_leaf() eventually else
there will be three different helpers which probably mean the same thing.

  reply index

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-22 15:41 Steven Price
2019-07-22 15:41 ` [PATCH v9 01/21] arc: mm: Add p?d_leaf() definitions Steven Price
2019-07-22 15:41 ` [PATCH v9 02/21] arm: " Steven Price
2019-07-22 15:41 ` [PATCH v9 03/21] arm64: " Steven Price
2019-07-22 15:41 ` [PATCH v9 04/21] mips: " Steven Price
2019-07-22 21:47   ` Paul Burton
2019-07-24 13:03     ` Steven Price
2019-07-22 15:41 ` [PATCH v9 05/21] powerpc: " Steven Price
2019-07-22 15:41 ` [PATCH v9 06/21] riscv: " Steven Price
2019-07-22 15:41 ` [PATCH v9 07/21] s390: " Steven Price
2019-07-22 15:41 ` [PATCH v9 08/21] sparc: " Steven Price
2019-07-22 15:41 ` [PATCH v9 09/21] x86: " Steven Price
2019-07-22 15:41 ` [PATCH v9 10/21] mm: Add generic p?d_leaf() macros Steven Price
2019-07-23  9:41   ` Mark Rutland
2019-07-24 13:48     ` Steven Price
2019-07-28 11:44     ` Anshuman Khandual
2019-07-29 11:38       ` Steven Price
2019-08-01  6:09         ` Anshuman Khandual
2019-08-01 12:22           ` Steven Price
2019-07-29 12:50       ` Mark Rutland
2019-08-01  6:13         ` Anshuman Khandual
2019-07-22 15:42 ` [PATCH v9 11/21] mm: pagewalk: Add p4d_entry() and pgd_entry() Steven Price
2019-07-23 10:14   ` Mark Rutland
2019-07-24 13:53     ` Steven Price
2019-07-24 14:09       ` Mark Rutland
2019-07-28 12:33   ` Anshuman Khandual
2019-07-29 12:17     ` Steven Price
2019-07-22 15:42 ` [PATCH v9 12/21] mm: pagewalk: Allow walking without vma Steven Price
2019-07-28 14:20   ` Anshuman Khandual
2019-07-29 12:29     ` Steven Price
2019-08-01  6:41       ` Anshuman Khandual
2019-07-22 15:42 ` [PATCH v9 13/21] mm: pagewalk: Add test_p?d callbacks Steven Price
2019-07-28 13:41   ` Anshuman Khandual
2019-07-29 12:34     ` Steven Price
2019-07-22 15:42 ` [PATCH v9 14/21] x86: mm: Don't display pages which aren't present in debugfs Steven Price
2019-07-22 15:42 ` [PATCH v9 15/21] x86: mm: Point to struct seq_file from struct pg_state Steven Price
2019-07-22 15:42 ` [PATCH v9 16/21] x86: mm+efi: Convert ptdump_walk_pgd_level() to take a mm_struct Steven Price
2019-07-22 15:42 ` [PATCH v9 17/21] x86: mm: Convert ptdump_walk_pgd_level_debugfs() to take an mm_struct Steven Price
2019-07-22 15:42 ` [PATCH v9 18/21] x86: mm: Convert ptdump_walk_pgd_level_core() " Steven Price
2019-07-22 15:42 ` [PATCH v9 19/21] mm: Add generic ptdump Steven Price
2019-07-23  9:57   ` Mark Rutland
2019-07-24 16:36     ` Steven Price
2019-07-29  2:59   ` Anshuman Khandual
2019-07-29 13:56     ` Steven Price
2019-07-22 15:42 ` [PATCH v9 20/21] x86: mm: Convert dump_pagetables to use walk_page_range Steven Price
2019-07-22 15:42 ` [PATCH v9 21/21] arm64: mm: Convert mm/dump.c to use walk_page_range() Steven Price
2019-07-23  6:39 ` [PATCH v9 00/21] Generic page walk and ptdump Anshuman Khandual
2019-07-24 13:35   ` Steven Price
2019-07-25  9:09     ` Anshuman Khandual
2019-07-25  9:30       ` Will Deacon
2019-07-26  6:03         ` Anshuman Khandual [this message]
2019-07-25 10:15       ` Steven Price
2019-07-23 10:16 ` Mark Rutland
2019-07-24 13:35   ` Steven Price
2019-07-24 13:57     ` Thomas Gleixner
2019-07-24 14:07       ` Mark Rutland
2019-07-24 14:18       ` Steven Price
2019-07-24 14:37         ` Thomas Gleixner
2019-07-28 11:20 ` Anshuman Khandual
2019-07-29 11:32   ` Steven Price
2019-07-31  9:27     ` Sven Schnelle
2019-07-31 11:18       ` Steven Price

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