Re: [PATCH] mm/thp: fix comments in transparent_hugepage_flags

[Jianguo Wu <wujianguo@huawei.com>]

On 2013/9/5 12:58, Wanpeng Li wrote:

> Hi Jianguo,
> On Thu, Sep 05, 2013 at 11:54:00AM +0800, Jianguo Wu wrote:
>> On 2013/9/5 11:37, Wanpeng Li wrote:
>>
>>> On Thu, Sep 05, 2013 at 11:04:22AM +0800, Jianguo Wu wrote:
>>>> Hi Wanpeng,
>>>>
>>>> On 2013/9/5 10:11, Wanpeng Li wrote:
>>>>
>>>>> Hi Jianguo,
>>>>> On Wed, Sep 04, 2013 at 09:30:22PM +0800, Jianguo Wu wrote:
>>>>>> Since commit d39d33c332(thp: enable direct defrag), defrag is enable
>>>>>> for all transparent hugepage page faults by default, not only in
>>>>>> MADV_HUGEPAGE regions.
>>>>>>
>>>>>> Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
>>>>>> ---
>>>>>> mm/huge_memory.c | 6 ++----
>>>>>> 1 file changed, 2 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index a92012a..abf047e 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -28,10 +28,8 @@
>>>>>>
>>>>>> /*
>>>>>>  * By default transparent hugepage support is enabled for all mappings
>>>>>
>>>>> This is also stale. TRANSPARENT_HUGEPAGE_ALWAYS is not configured by default in
>>>>> order that avoid to risk increase the memory footprint of applications w/o a 
>>>>> guaranteed benefit.
>>>>>
>>>>
>>>> Right, how about this:
>>>>
>>>> By default transparent hugepage support is disabled in order that avoid to risk
>>>
>>> I don't think it's disabled. TRANSPARENT_HUGEPAGE_MADVISE is configured
>>> by default.
>>>
>>
>> Hi Wanpeng,
>>
>> We have TRANSPARENT_HUGEPAGE and TRANSPARENT_HUGEPAGE_ALWAYS/TRANSPARENT_HUGEPAGE_MADVISE,
>> TRANSPARENT_HUGEPAGE_ALWAYS or TRANSPARENT_HUGEPAGE_MADVISE is configured only if TRANSPARENT_HUGEPAGE
>> is configured.
>>
>> By default, TRANSPARENT_HUGEPAGE=n, and TRANSPARENT_HUGEPAGE_ALWAYS is configured when TRANSPARENT_HUGEPAGE=y.
>>
>> commit 13ece886d9(thp: transparent hugepage config choice):
>>
>> config TRANSPARENT_HUGEPAGE
>> -       bool "Transparent Hugepage Support" if EMBEDDED
>> +       bool "Transparent Hugepage Support"
>>        depends on X86 && MMU
>> -       default y
>>
>> +choice
>> +       prompt "Transparent Hugepage Support sysfs defaults"
>> +       depends on TRANSPARENT_HUGEPAGE
>> +       default TRANSPARENT_HUGEPAGE_ALWAYS
>>
> 
> mmotm tree:
> 
> grep 'TRANSPARENT_HUGEPAGE' .config
> CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
> CONFIG_TRANSPARENT_HUGEPAGE=y
> # CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS is not set
> CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y
> 
> distro:
> 
> grep 'TRANSPARENT_HUGEPAGE' config-3.8.0-26-generic 
> CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
> CONFIG_TRANSPARENT_HUGEPAGE=y
> # CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS is not set
> CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y
> 

Hi Wanpeng,

I'm a little confused, at mm/Kconfig, TRANSPARENT_HUGEPAGE is not configured by default.

and in x86_64, linus tree:

$make defconfig
$grep 'TRANSPARENT_HUGEPAGE' .config
CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE=y
# CONFIG_TRANSPARENT_HUGEPAGE is not set

Do i misunderstand something herei 1/4 ?

Thanks

> 
>> Thanks,
>> Jianguo Wu
>>
>>> Regards,
>>> Wanpeng Li 
>>>
>>>> increase the memory footprint of applications w/o a guaranteed benefit, and
>>>> khugepaged scans all mappings when transparent hugepage enabled.
>>>> Defrag is invoked by khugepaged hugepage allocations and by page faults for all
>>>> hugepage allocations.
>>>>
>>>> Thanks,
>>>> Jianguo Wu
>>>>
>>>>> Regards,
>>>>> Wanpeng Li 
>>>>>
>>>>>> - * and khugepaged scans all mappings. Defrag is only invoked by
>>>>>> - * khugepaged hugepage allocations and by page faults inside
>>>>>> - * MADV_HUGEPAGE regions to avoid the risk of slowing down short lived
>>>>>> - * allocations.
>>>>>> + * and khugepaged scans all mappings. Defrag is invoked by khugepaged
>>>>>> + * hugepage allocations and by page faults for all hugepage allocations.
>>>>>>  */
>>>>>> unsigned long transparent_hugepage_flags __read_mostly =
>>>>>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS
>>>>>> -- 
>>>>>> 1.8.1.2
>>>>>>
>>>>>> --
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>>>>>
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>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>> .
>>>
>>
>>
> 
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