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

[Michal Hocko <mhocko@suse.com>]

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@bytedance.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>
> Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
> Tested-by: Chen Huang <chenhuang5@huawei.com>
> Tested-by: Bodeddula Balasubramaniam <bodeddub@amazon.com>
> ---
>  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
			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
-- 
Michal Hocko
SUSE Labs