On Thu, Oct 1, 2020 at 6:41 AM Mike Kravetz [off-list ref] wrote:

On 9/15/20 5:59 AM, Muchun Song wrote:

quoted

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.

quoted

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

quoted

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 <redacted>
---
 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