Re: [PATCH V2] mm: Introduce GFP_PGTABLE

From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: mark.rutland@arm.com, linux-sh@vger.kernel.org,
	peterz@infradead.org, catalin.marinas@arm.com,
	dave.hansen@linux.intel.com, will.deacon@arm.com,
	christoffer.dall@arm.com, linux-mm@kvack.org,
	linux-riscv@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
	mpe@ellerman.id.au, greentime@andestech.com,
	linux@armlinux.org.uk, mingo@redhat.com, vbabka@suse.cz,
	rientjes@google.com, palmer@sifive.com,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	marc.zyngier@arm.com, rppt@linux.vnet.ibm.com,
	shakeelb@google.com, kirill@shutemov.name, tglx@linutronix.de,
	linux-arm-kernel@lists.infradead.org, ard.biesheuvel@linaro.org,
	robin.murphy@arm.com, steve.capper@arm.com,
	linux-kernel@vger.kernel.org, james.morse@arm.com,
	aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE
Date: Wed, 16 Jan 2019 14:33:02 +0100	[thread overview]
Message-ID: <20190116133302.GN24149@dhcp22.suse.cz> (raw)
In-Reply-To: <20190116131827.GH6310@bombadil.infradead.org>

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

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