linux-kernel - Re: [PATCH v9 00/21] Generic page walk and ptdump

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Message-ID: <20190725093036.dzn6uulcihhkohm2@willie-the-truck>
Date:   Thu, 25 Jul 2019 10:30:37 +0100
From:   Will Deacon <will@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Steven Price <steven.price@....com>, linux-mm@...ck.org,
        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>,
        linux-kernel@...r.kernel.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arm-kernel@...ts.infradead.org,
        "Liang, Kan" <kan.liang@...ux.intel.com>
Subject: Re: [PATCH v9 00/21] Generic page walk and ptdump

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.

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.

> > 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.

> 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.

I don't usually care much about naming, but in this case I really find
the status quo needlessly confusing.

Will