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Subject: Re: [V2] powerpc/Kconfig: Update config option based on page size.
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
 Michael Ellerman <mpe@ellerman.id.au>, Rashmica Gupta <rashmicy@gmail.com>,
 linuxppc-dev@lists.ozlabs.org
References: <3qk8fY48r6z9t45@ozlabs.org> <87y4893ar2.fsf@skywalker.in.ibm.com>
From: Balbir Singh <bsingharora@gmail.com>
Message-ID: <5716BAB2.4070807@gmail.com>
Date: Wed, 20 Apr 2016 09:09:38 +1000
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On 20/04/16 00:59, Aneesh Kumar K.V wrote:
> Michael Ellerman <mpe@ellerman.id.au> writes:
> 
>> On Fri, 2016-19-02 at 05:38:47 UTC, Rashmica Gupta wrote:
>>> Currently on PPC64 changing kernel pagesize from 4K to 64K leaves
>>> FORCE_MAX_ZONEORDER set to 13 - which produces a compile error.
>>>
>> ...
>>> So, update the range of FORCE_MAX_ZONEORDER from 9-64 to 8-9 for 64K pages
>>> and from 13-64 to 9-13 for 4K pages.
>>>
>>> Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
>>> Reviewed-by: Balbir Singh <bsingharora@gmail.com>
>>
>> Applied to powerpc next, thanks.
>>
>> https://git.kernel.org/powerpc/c/a7ee539584acf4a565b7439cea
>>
> 
> HPAGE_PMD_ORDER is not something we should check w.r.t 4k linux page
> size. We do have the below constraint w.r.t hugetlb pages
> 
> static inline bool hstate_is_gigantic(struct hstate *h)
> {
> 	return huge_page_order(h) >= MAX_ORDER;
> }
> 
> That require MAX_ORDER to be greater than 12.
> 

The build will fail for MAX_ZONEORDER beyond the specified limits.
MAX_ORDER > 12 for what page size?

My understanding is this

1. gigantic refers to the fact the regular allocators cannot allocate
this page
2. Use alloc_contig_range() with CONFIG_CMA for gigantic pages

I could be wrong

> Did we test hugetlbfs 4k config with this patch ? Will it work if we
> start marking hugepage as gigantic page ?

Nope.. I did not

Thanks for the review!
Balbir Singh