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

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Message-ID: <87tvi74nfx.fsf@concordia.ellerman.id.au>
Date:   Thu, 17 Jan 2019 20:56:34 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-sh@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, linux-riscv@...ts.infradead.org,
        linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, tglx@...utronix.de, mingo@...hat.com,
        dave.hansen@...ux.intel.com, peterz@...radead.org,
        christoffer.dall@....com, marc.zyngier@....com,
        kirill@...temov.name, rppt@...ux.vnet.ibm.com,
        ard.biesheuvel@...aro.org, mark.rutland@....com,
        steve.capper@....com, james.morse@....com, robin.murphy@....com,
        aneesh.kumar@...ux.ibm.com, vbabka@...e.cz, shakeelb@...gle.com,
        rientjes@...gle.com, palmer@...ive.com, greentime@...estech.com
Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE

Matthew Wilcox <willy@...radead.org> writes:

> On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote:
>> On Wed 16-01-19 11:51:32, Anshuman Khandual wrote:
>> > All architectures have been defining their own PGALLOC_GFP as (GFP_KERNEL |
>> > __GFP_ZERO) and using it for allocating page table pages. This causes some
>> > code duplication which can be easily avoided. GFP_KERNEL allocated and
>> > cleared out pages (__GFP_ZERO) are required for page tables on any given
>> > architecture. This creates a new generic GFP flag flag which can be used
>> > for any page table page allocation. Does not cause any functional change.
>> > 
>> > GFP_PGTABLE is being added into include/asm-generic/pgtable.h which is the
>> > generic page tabe header just to prevent it's potential misuse as a general
>> > allocation flag if included in include/linux/gfp.h.
>> 
>> I haven't reviewed the patch yet but I am wondering whether this is
>> really worth it without going all the way down to unify the common code
>> and remove much more code duplication. Or is this not possible for some
>> reason?
>
> Exactly what I suggested doing in response to v1.
>
> Also, the approach taken here is crazy.  x86 has a feature that no other
> architecture has bothered to implement yet -- accounting page tables
> to the process.

powerpc does __GFP_ACCOUNT for user page tables too. Shamelessly cribbed
from the x86 version of course :)

cheers