Re: [PATCH v18 6/9] mm: hugetlb: add a kernel parameter hugetlb_free_vmemmap

[Randy Dunlap]

On 3/10/21 7:37 AM, Michal Hocko wrote:
> On Mon 08-03-21 18:28:04, Muchun Song wrote:
>> Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
>> freeing unused vmemmap pages associated with each hugetlb page on boot.
>>
>> We disables PMD mapping of vmemmap pages for x86-64 arch when this
>> feature is enabled. Because vmemmap_remap_free() depends on vmemmap
>> being base page mapped.
>>
>> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
>> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
>> Reviewed-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
>> Reviewed-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> Tested-by: Chen Huang <chenhuang5@xxxxxxxxxx>
>> Tested-by: Bodeddula Balasubramaniam <bodeddub@xxxxxxxxxx>
>> ---
>>  Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
>>  Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
>>  arch/x86/mm/init_64.c                           |  8 ++++++--
>>  include/linux/hugetlb.h                         | 19 +++++++++++++++++++
>>  mm/hugetlb_vmemmap.c                            | 24 ++++++++++++++++++++++++
>>  5 files changed, 66 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 04545725f187..de91d54573c4 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -1557,6 +1557,20 @@
>>  			Documentation/admin-guide/mm/hugetlbpage.rst.
>>  			Format: size[KMG]
>>  
>> +	hugetlb_free_vmemmap=
>> +			[KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,
>> +			this controls freeing unused vmemmap pages associated
>> +			with each HugeTLB page. When this option is enabled,
>> +			we disable PMD/huge page mapping of vmemmap pages which
>> +			increase page table pages. So if a user/sysadmin only
>> +			uses a small number of HugeTLB pages (as a percentage
>> +			of system memory), they could end up using more memory
>> +			with hugetlb_free_vmemmap on as opposed to off.
>> +			Format: { on | off (default) }
> 
> Please note this is an admin guide and for those this seems overly low
> level. I would use something like the following
> 			[KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> 			enabled.
> 			Allows heavy hugetlb users to free up some more
> 			memory (6 * PAGE_SIZE for each 2MB hugetlb
> 			page).
> 			This feauture is not free though. Large page
> 			tables are not use to back vmemmap pages which

			       are not used

> 			can lead to a performance degradation for some
> 			workloads. Also there will be memory allocation
> 			required when hugetlb pages are freed from the
> 			pool which can lead to corner cases under heavy
> 			memory pressure.
>> +
>> +			on:  enable the feature
>> +			off: disable the feature
>> +
>>  	hung_task_panic=
>>  			[KNL] Should the hung task detector generate panics.
>>  			Format: 0 | 1


-- 
~Randy