linux-kernel - Re: [PATCH v2 03/13] mm: Add generic p?d

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Date:   Fri, 22 Feb 2019 10:21:40 +0000
From:   Steven Price <steven.price@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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, 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 v2 03/13] mm: Add generic p?d_large() macros

On 21/02/2019 21:06, Kirill A. Shutemov wrote:
> On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote:
>>>> Note that in terms of the new page walking code, these new defines are
>>>> only used when walking a page table without a VMA (which isn't currently
>>>> done), so architectures which don't use p?d_large currently will work
>>>> fine with the generic versions. They only need to provide meaningful
>>>> definitions when switching to use the walk-without-a-VMA functionality.
>>>
>>> How other architectures would know that they need to provide the helpers
>>> to get walk-without-a-VMA functionality? This looks very fragile to me.
>>
>> Yes, you've got a good point there. This would apply to the p?d_large
>> macros as well - any arch which (inadvertently) uses the generic version
>> is likely to be fragile/broken.
>>
>> I think probably the best option here is to scrap the generic versions
>> altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which
>> would enable the new functionality to those arches that opt-in. Do you
>> think this would be less fragile?
> 
> These helpers are useful beyond pagewalker.
> 
> Can we actually do some grinding and make *all* archs to provide correct
> helpers? Yes, it's tedious, but not that bad.
> 
> I think we could provide generic helpers for folded levels in
> <asm-generic/pgtable-nop?d.h> and rest has to be provided by the arch.
> Architectures that support only 2 level paging would need to provide
> pgd_large(), with 3 -- pmd_large() and so on.

Fair enough, I'll have a go and hopefully people will be able to correct
it if I make any mistakes - I'm certainly not going to be able to test
all architectures myself.

Steve