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

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Date:   Wed, 16 Jan 2019 14:33:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.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,
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        kvmarm@...ts.cs.columbia.edu, linux-riscv@...ts.infradead.org,
        linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, mpe@...erman.id.au, 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,
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        greentime@...estech.com
Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE

On Wed 16-01-19 05:18:27, Matthew Wilcox wrote:
> On Wed, Jan 16, 2019 at 06:42:22PM +0530, Anshuman Khandual wrote:
> > On 01/16/2019 06:00 PM, Matthew Wilcox wrote:
> > > 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.  Yet instead of spreading that goodness to all other
> > > architectures, Anshuman has gone to more effort to avoid doing that.
> > 
> > The basic objective for this patch is to create a common minimum allocation
> > flag that can be used by architectures but that still allows archs to add
> > on additional constraints if they see fit. This patch does not intend to
> > change functionality for any arch.
> 
> 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.

Agreed!

-- 
Michal Hocko
SUSE Labs