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

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;
}

--
2.20.1
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