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

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Date:   Wed, 16 Jan 2019 13:34:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        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, mpe@...erman.id.au, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com,
        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

On Wed, Jan 16, 2019 at 04:30:18AM -0800, 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.

Also, s390 (of course, who else) has a fun custom page-table allocator
for reasons that don't apply to many other archs.