Re: [External] Re: [RFC PATCH 05/24] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Oct 1, 2020 at 6:41 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 9/15/20 5:59 AM, Muchun Song wrote:
> > If the size of hugetlb page is 2MB, we need 512 struct page structures
> > (8 pages) to be associated with it. As far as I know, we only use the
> > first 3 struct page structures and only read the compound_dtor members
>
> Actually, the first 4 pages can be used if CONFIG_CGROUP_HUGETLB.

Right, thanks.

> /*
>  * Minimum page order trackable by hugetlb cgroup.
>  * At least 4 pages are necessary for all the tracking information.
>  * The second tail page (hpage[2]) is the fault usage cgroup.
>  * The third tail page (hpage[3]) is the reservation usage cgroup.
>  */
> #define HUGETLB_CGROUP_MIN_ORDER        2
>
> However, this still easily fits within the first page of struct page
> structures.
>
> > of the remaining struct page structures. For tail page, the value of
> > compound_dtor is the same. So we can reuse first tail page. We map the
> > virtual addresses of the remaining 6 tail pages to the first tail page,
> > and then free these 6 pages. Therefore, we need to reserve at least 2
> > pages as vmemmap areas.
>
> I got confused the first time I read the above sentences.  Perhaps it
> should be more explicit with something like:
>
> For tail pages, the value of compound_dtor is the same. So we can reuse
> first page of tail page structs. We map the virtual addresses of the
> remaining 6 pages of tail page structs to the first tail page struct,
> and then free these 6 pages. Therefore, we need to reserve at least 2
> pages as vmemmap areas.

Sorry for my poor English. Thanks for your suggestions. I can apply this.

>
> It still does not sound great, but hopefully avoids some confusion.
> --
> Mike Kravetz
>
> > So we introduce a new nr_free_vmemmap_pages field in the hstate to
> > indicate how many vmemmap pages associated with a hugetlb page that we
> > can free to buddy system.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/hugetlb.h |  3 +++
> >  mm/hugetlb.c            | 35 +++++++++++++++++++++++++++++++++++
> >  2 files changed, 38 insertions(+)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index d5cc5f802dd4..eed3dd3bd626 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -492,6 +492,9 @@ struct hstate {
> >       unsigned int nr_huge_pages_node[MAX_NUMNODES];
> >       unsigned int free_huge_pages_node[MAX_NUMNODES];
> >       unsigned int surplus_huge_pages_node[MAX_NUMNODES];
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +     unsigned int nr_free_vmemmap_pages;
> > +#endif
> >  #ifdef CONFIG_CGROUP_HUGETLB
> >       /* cgroup control files */
> >       struct cftype cgroup_files_dfl[7];
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 81a41aa080a5..f1b2b733b49b 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1292,6 +1292,39 @@ static inline void destroy_compound_gigantic_page(struct page *page,
> >                                               unsigned int order) { }
> >  #endif
> >
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +#define RESERVE_VMEMMAP_NR   2U
> > +
> > +static inline unsigned int nr_free_vmemmap(struct hstate *h)
> > +{
> > +     return h->nr_free_vmemmap_pages;
> > +}
> > +
> > +static void __init hugetlb_vmemmap_init(struct hstate *h)
> > +{
> > +     unsigned int order = huge_page_order(h);
> > +     unsigned int vmemmap_pages;
> > +
> > +     vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
> > +     /*
> > +      * The head page and the first tail page not free to buddy system,
> > +      * the others page will map to the first tail page. So there are
> > +      * (@vmemmap_pages - RESERVE_VMEMMAP_NR) pages can be freed.
> > +      */
> > +     if (vmemmap_pages > RESERVE_VMEMMAP_NR)
> > +             h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> > +     else
> > +             h->nr_free_vmemmap_pages = 0;
> > +
> > +     pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
> > +             h->nr_free_vmemmap_pages, h->name);
> > +}
> > +#else
> > +static inline void hugetlb_vmemmap_init(struct hstate *h)
> > +{
> > +}
> > +#endif
> > +
> >  static void update_and_free_page(struct hstate *h, struct page *page)
> >  {
> >       int i;
> > @@ -3285,6 +3318,8 @@ void __init hugetlb_add_hstate(unsigned int order)
> >       snprintf(h->name, HSTATE_NAME_LEN, "hugepages-%lukB",
> >                                       huge_page_size(h)/1024);
> >
> > +     hugetlb_vmemmap_init(h);
> > +
> >       parsed_hstate = h;
> >  }
> >
> >



-- 
Yours,
Muchun