On Thu, Mar 11, 2021 at 1:16 AM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>
> 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
Thanks.
>
> > 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
>
