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

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Message-ID: <f9f333a5-5533-996a-dc8e-1ff1096c1d19@arm.com>
Date:   Mon, 14 Jan 2019 09:30:55 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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@...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,
        robin.murphy@....com, aneesh.kumar@...ux.ibm.com, vbabka@...e.cz,
        shakeelb@...gle.com, rientjes@...gle.com
Subject: Re: [PATCH] mm: Introduce GFP_PGTABLE



On 01/13/2019 11:05 PM, Michal Hocko wrote:
> On Sat 12-01-19 15:56:38, 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.
> 
> I agree that some unification is due but GFP_PGTABLE is not something to
> expose in generic gfp.h IMHO. It just risks an abuse. I would be looking

Why would you think that it risks an abuse ? It does not create new semantics
of allocation in the buddy. Its just uses existing GFP_KERNEL allocation which
is then getting zeroed out. The risks (if any) is exactly same as GFP_KERNEL.

> at providing asm-generic implementation and reuse it to remove the code

Does that mean GFP_PGTABLE can be created but not in gfp.h but in some other
memory related header file ?

> duplication. But I haven't tried that to know that it will work out due
> to small/subtle differences between arches.

IIUC from the allocation perspective GFP_ACCOUNT is the only thing which gets
added with GFP_PGTABLE for user page table for memcg accounting purpose. There
does not seem to be any other differences unless I am missing something.