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

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Message-ID: <d074a7aa-9582-b95a-dce0-d95ac3d3c949@arm.com>
Date:   Wed, 16 Jan 2019 18:57:13 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Michal Hocko <mhocko@...nel.org>,
        Matthew Wilcox <willy@...radead.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-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,
        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 01/16/2019 06:14 PM, Michal Hocko wrote:
> On Wed 16-01-19 04:30:18, 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.
> 
> Yes, I believe the only reason this is x86 only is that each arch would
> have to be tweaked separately. So a cleanup in _that_ regard would be
> helpful. There is no real reason to have ptes accounted only for x86.
> There might be some exceptions but well, our asm-generic allows to opt
> in for generic implementation or override it with a special one. The
> later should be an exception rather than the rule.

Fair enough. So we seem to have agreement over __GFP_ACCOUNT for user page
tables but not for the kernel. But should we accommodate __GFP_RETRY_MAYFAIL
or drop them altogether (including multi order allocation requests) ?