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

[Muchun Song]

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
>