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

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Mar 10, 2021 at 11:37 PM Michal Hocko <mhocko@suse.com> 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@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

OK.

> 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.

Very thanks. I will update this.

> > +
> > +                     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