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
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
