linux-kernel - Re: [PATCH v2] powerpc/mm: Fix growth direction for hugepages mmaps with slice

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Date:   Wed, 17 Jan 2018 08:49:03 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Christophe LEROY <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] powerpc/mm: Fix growth direction for hugepages mmaps
 with slice



On 01/16/2018 10:18 PM, Christophe LEROY wrote:
> 
> 
> Le 16/01/2018 à 17:03, Aneesh Kumar K.V a écrit :
>> Christophe Leroy <christophe.leroy@....fr> writes:
>>
>>> An application running with libhugetlbfs fails to allocate
>>> additional pages to HEAP due to the hugemap being done
>>> inconditionally as topdown mapping:
>>>
>>> mmap(0x10080000, 1572864, PROT_READ|PROT_WRITE, 
>>> MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0) = 0x73e80000
>>> [...]
>>> mmap(0x74000000, 1048576, PROT_READ|PROT_WRITE, 
>>> MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d80000
>>> munmap(0x73d80000, 1048576)             = 0
>>> [...]
>>> mmap(0x74000000, 1572864, PROT_READ|PROT_WRITE, 
>>> MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d00000
>>> munmap(0x73d00000, 1572864)             = 0
>>> [...]
>>> mmap(0x74000000, 1572864, PROT_READ|PROT_WRITE, 
>>> MAP_PRIVATE|MAP_ANONYMOUS|0x40000, -1, 0x180000) = 0x73d00000
>>> munmap(0x73d00000, 1572864)             = 0
>>> [...]
>>>
>>
>> Can you explain the failure details above. I am not sure I understand
>> what to read from the above output.
> 
> libhugetlbfs first requests an area of size 1.5Mbytes, at address 
> 0x10080000
> mmap() returns an area at address 0x73e80000
> 
> Then libhugetlbfs requests an additional area on top of that, ie at 
> address 0x74000000, to expand the heap.
> But mmap() returns an area at address 0x73d80000, ie under the previous 
> area.
> 


Can you share the test details?. Why does it not fail on book3s64? We 
use topdown search with book3s64.

> This is not the behaviour when using the generic (ie without mm_slices) 
> hugepages code, and this is not what libhugetlbfs expects for expending 
> the heap.
> 
>

-aneesh