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

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Message-ID: <20190221210618.voyfs5cnafpvgedh@kshutemo-mobl1>
Date:   Fri, 22 Feb 2019 00:06:18 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Steven Price <steven.price@....com>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
        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 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.

-- 
 Kirill A. Shutemov