Re: [External] Re: [PATCH v5 17/21] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

[Muchun Song <songmuchun@bytedance.com>]

On Fri, Nov 20, 2020 at 4:22 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 20-11-20 14:43:21, Muchun Song wrote:
> > Add a kernel parameter hugetlb_free_vmemmap to disable the feature of
> > freeing unused vmemmap pages associated with each hugetlb page on boot.
>
> As replied to the config patch. This is fine but I would argue that the
> default should be flipped. Saving memory is nice but it comes with
> overhead and therefore should be an opt-in. The config option should
> only guard compile time dependencies not a user choice.

Got it. The default will be flipped in the next version. Thanks.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
> >  Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
> >  mm/hugetlb_vmemmap.c                            | 21 +++++++++++++++++++++
> >  3 files changed, 33 insertions(+)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 5debfe238027..ccf07293cb63 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1551,6 +1551,15 @@
> >                       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.
> > +                     Format: { on (default) | off }
> > +
> > +                     on:  enable the feature
> > +                     off: disable the feature
> > +
> >       hung_task_panic=
> >                       [KNL] Should the hung task detector generate panics.
> >                       Format: 0 | 1
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..7d6129ee97dd 100644
> > --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> > +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> > @@ -145,6 +145,9 @@ default_hugepagesz
> >
> >       will all result in 256 2M huge pages being allocated.  Valid default
> >       huge page size is architecture dependent.
> > +hugetlb_free_vmemmap
> > +     When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set, this disables freeing
> > +     unused vmemmap pages associated each HugeTLB page.
> >
> >  When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
> >  indicates the current number of pre-allocated huge pages of the default size.
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 3629165d8158..c958699d1393 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -144,6 +144,22 @@ static inline bool vmemmap_pmd_huge(pmd_t *pmd)
> >  }
> >  #endif
> >
> > +static bool hugetlb_free_vmemmap_disabled __initdata;
> > +
> > +static int __init early_hugetlb_free_vmemmap_param(char *buf)
> > +{
> > +     if (!buf)
> > +             return -EINVAL;
> > +
> > +     if (!strcmp(buf, "off"))
> > +             hugetlb_free_vmemmap_disabled = true;
> > +     else if (strcmp(buf, "on"))
> > +             return -EINVAL;
> > +
> > +     return 0;
> > +}
> > +early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
> > +
> >  static inline unsigned int vmemmap_pages_per_hpage(struct hstate *h)
> >  {
> >       return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
> > @@ -541,6 +557,11 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >       unsigned int order = huge_page_order(h);
> >       unsigned int vmemmap_pages;
> >
> > +     if (hugetlb_free_vmemmap_disabled) {
> > +             pr_info("disable free vmemmap pages for %s\n", h->name);
> > +             return;
> > +     }
> > +
> >       vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
> >       /*
> >        * The head page and the first tail page are not to be freed to buddy
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun