linux-kernel - Re: [PATCH V2] mm: Introduce GFP

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Date:   Wed, 16 Jan 2019 06:20:14 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        mark.rutland@....com, linux-sh@...r.kernel.org,
        peterz@...radead.org, catalin.marinas@....com,
        dave.hansen@...ux.intel.com, will.deacon@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux@...linux.org.uk, mingo@...hat.com, vbabka@...e.cz,
        rientjes@...gle.com, palmer@...ive.com, greentime@...estech.com,
        marc.zyngier@....com, rppt@...ux.vnet.ibm.com, shakeelb@...gle.com,
        kirill@...temov.name, tglx@...utronix.de,
        Michal Hocko <mhocko@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, ard.biesheuvel@...aro.org,
        robin.murphy@....com, steve.capper@....com,
        christoffer.dall@....com, james.morse@....com,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE

On Wed, Jan 16, 2019 at 02:47:16PM +0100, Christophe Leroy wrote:
> Le 16/01/2019 à 14:18, Matthew Wilcox a écrit :
> > I disagree with your objective.  Making more code common is a great idea,
> > but this patch is too unambitious.  We should be heading towards one or
> > two page table allocation functions instead of having every architecture do
> > its own thing.
> > 
> > So start there.  Move the x86 function into common code and convert one
> > other architecture to use it too.
> 
> Are we talking about pte_alloc_one_kernel() and pte_alloc_one() ?
> 
> I'm not sure x86 function is the best common one, as it seems to allocate a
> multiple of PAGE_SIZE only.

And that's the common case.  Most architectures use a single page for at
least one level of the pte/pmd/pud/p4d/pgd hierarchy.  Some use multiple
pages and some use a fraction of a page.

> Some arches like powerpc use pagetables which are smaller than a page, for
> instance powerpc 8xx uses 4k pagetables even with 16k pages, which means a
> single page can be used by 4 pagetables.

Those can be added later.  Note I said "one or two", and that's what I
had in mind; I think we want one function that allocates just a page
and another that allocates a page fragment.  Then we can have a good
discussion about what method we use; s390 and ppc use different techniques
today and there's really no good reason for that.