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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

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@c-s.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