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. /* * 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. 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 <[email protected]> > --- > 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; > } > >
