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Subject: Re: [PATCH] mm: Introduce GFP_PGTABLE
To:     Michal Hocko <mhocko@kernel.org>
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 <20190113173555.GC1578@dhcp22.suse.cz>
From:   Anshuman Khandual <anshuman.khandual@arm.com>
Message-ID: <f9f333a5-5533-996a-dc8e-1ff1096c1d19@arm.com>
Date:   Mon, 14 Jan 2019 09:30:55 +0530
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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.