linux-kernel - Re: [PATCH 1/3] powerpc/32: Fix hugepage allocation on 8xx at hint address

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Message-ID: <003fc691-074a-9226-3ab2-69ff4a76f871@c-s.fr>
Date:   Tue, 16 Jan 2018 17:53:07 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/3] powerpc/32: Fix hugepage allocation on 8xx at hint
 address



Le 16/01/2018 à 17:43, Aneesh Kumar K.V a écrit :
> 
> 
> 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@....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?

Exactly

And also avoid hugetlb_get_unmapped_area() accepting an hint address in 
that range when we already have a regular mapping in that range.

Christophe