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

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Message-ID: <48f81a76-2b2d-5060-d9bc-c42d5b5975c3@arm.com>
Date:   Mon, 14 Jan 2019 09:58:14 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     mark.rutland@....com, mhocko@...e.com, linux-sh@...r.kernel.org,
        peterz@...radead.org, catalin.marinas@....com,
        dave.hansen@...ux.intel.com, will.deacon@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kvmarm@...ts.cs.columbia.edu, linux@...linux.org.uk,
        mingo@...hat.com, vbabka@...e.cz, rientjes@...gle.com,
        marc.zyngier@....com, rppt@...ux.vnet.ibm.com, shakeelb@...gle.com,
        kirill@...temov.name, tglx@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, ard.biesheuvel@...aro.org,
        robin.murphy@....com, steve.capper@....com,
        christoffer.dall@....com, james.morse@....com,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] mm: Introduce GFP_PGTABLE



On 01/12/2019 09:19 PM, Matthew Wilcox wrote:
> On Sat, Jan 12, 2019 at 02:49:29PM +0100, Christophe Leroy wrote:
>> As far as I can see,
>>
>> #define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)
>>
>> So what's the difference between:
>>
>> (GFP_KERNEL_ACCOUNT | __GFP_ZERO) & ~__GFP_ACCOUNT
>>
>> and
>>
>> (GFP_KERNEL | __GFP_ZERO) & ~__GFP_ACCOUNT
> 
> Nothing.  But there's a huge difference in the other parts of that same
> file where GFP_ACCOUNT is _not_ used.
> 
> I think this unification is too small to bother with.  Something I've
> had on my todo list for some time and have not done anything about
> is to actually unify all of the architecture pte/pmd/... allocations.
> There are tricks some architectures use that others would benefit from.

Sure. Could you please elaborate on this ?

Invariably all kernel pgtable page allocations should use GFP_PGTABLE and
all user page table allocation should use (GFP_PGTABLE | __GFP_ACCOUNT).
Ideally there should be default generic functions user_pgtable_gfp() or
kernel_pgtable_gfp() returning these values. Overrides can be provided if
an arch still wants some more control.