Subject: Re: [PATCH 1/3] powerpc/32: Fix hugepage allocation on 8xx at hint
 address
To: Christophe LEROY <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Scott Wood <oss@buserror.net>,
        Nicholas Piggin <npiggin@gmail.com>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <9a5dadc10f88e2fc0ac9fb5d18c5424df33f3f4c.1515169256.git.christophe.leroy@c-s.fr>
 <876081by7g.fsf@linux.vnet.ibm.com>
 <b5e34a5b-94b8-9405-929d-d35dde3fcb3b@c-s.fr>
From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Date: Tue, 16 Jan 2018 22:13:39 +0530
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On 01/16/2018 10:01 PM, Christophe LEROY wrote:
> 
> 
> Le 16/01/2018 à 16:49, Aneesh Kumar K.V a écrit :
>> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>>
>>> When an app has some regular pages allocated (e.g. see below) and tries
>>> to mmap() a huge page at a hint address covered by the same PMD entry,
>>> the kernel accepts the hint allthough the 8xx cannot handle different
>>> page sizes in the same PMD entry.
>>
>>
>> So that is a bug in get_unmapped_area function that you are using and
>> you want to fix that by using the slice code. Can you describe here what
>> the allocation restrictions are w.r.t 8xx? Do they have segments and
>> base page size like hash64?
> 
> I don't think it is a bug in get_unmapped_area() that is used by 
> default. It is that some HW do support mixing any page size in the same 
> page table (eg BOOK3E ?), but the 8xx doesn't.
> In the 8xx, the page size is defined in the PGD entry, then all pages 
> defined in a given page table pointed by a PGD entry have the same size.
> 
> So it is similar to segments if you consider each PGD entry as a kind of 
> segment
> 

so IIUC, hugepd format encodes the page size details and that require us 
to ensure that all the address range mapped at that hupge_pd entry is of 
same page size? Hence we want to avoid mmap handing over an address in 
that range when we already have a hugetlb mapping in that range?


-aneesh