[PATCH v8 05/12] mm/hugetlb: Defer freeing of HugeTLB pages

Muchun Song
2020-12-10T04:05:31Z

In the subsequent patch, we will allocate the vmemmap pages when free
HugeTLB pages. But update_and_free_page() is called from a non-task
context(and hold hugetlb_lock), so we can defer the actual freeing in
a workqueue to prevent use GFP_ATOMIC to allocate the vmemmap pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
mm/hugetlb.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++----
mm/hugetlb_vmemmap.c | 12 --------
mm/hugetlb_vmemmap.h | 17 ++++++++++++
3 files changed, 88 insertions(+), 18 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 140135fc8113..0ff9b90e524f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1292,15 +1292,76 @@ static inline void destroy_compound_gigantic_page(struct page *page,
unsigned int order) { }
#endif

-static void update_and_free_page(struct hstate *h, struct page *page)
+static void __free_hugepage(struct hstate *h, struct page *page);
+
+/*
+ * As update_and_free_page() is be called from a non-task context(and hold
+ * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
+ * use GFP_ATOMIC to allocate a lot of vmemmap pages.
+ *
+ * update_hpage_vmemmap_workfn() locklessly retrieves the linked list of
+ * pages to be freed and frees them one-by-one. As the page->mapping pointer
+ * is going to be cleared in update_hpage_vmemmap_workfn() anyway, it is
+ * reused as the llist_node structure of a lockless linked list of huge
+ * pages to be freed.
+ */
+static LLIST_HEAD(hpage_update_freelist);
+
+static void update_hpage_vmemmap_workfn(struct work_struct *work)
{
- int i;
+ struct llist_node *node;
+ struct page *page;
+
+ node = llist_del_all(&hpage_update_freelist);

+ while (node) {
+ page = container_of((struct address_space **)node,
+ struct page, mapping);
+ node = node->next;
+ page->mapping = NULL;
+ __free_hugepage(page_hstate(page), page);
+
+ cond_resched();
+ }
+}
+static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
+
+static inline void __update_and_free_page(struct hstate *h, struct page *page)
+{
+ /* No need to allocate vmemmap pages */
+ if (!free_vmemmap_pages_per_hpage(h)) {
+ __free_hugepage(h, page);
+ return;
+ }
+
+ /*
+ * Defer freeing to avoid using GFP_ATOMIC to allocate vmemmap
+ * pages.
+ *
+ * Only call schedule_work() if hpage_update_freelist is previously
+ * empty. Otherwise, schedule_work() had been called but the workfn
+ * hasn't retrieved the list yet.
+ */
+ if (llist_add((struct llist_node *)&page->mapping,
+ &hpage_update_freelist))
+ schedule_work(&hpage_update_work);
+}
+
+static void update_and_free_page(struct hstate *h, struct page *page)
+{
if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
return;

h->nr_huge_pages--;
h->nr_huge_pages_node[page_to_nid(page)]--;
+
+ __update_and_free_page(h, page);
+}
+
+static void __free_hugepage(struct hstate *h, struct page *page)
+{
+ int i;
+
for (i = 0; i < pages_per_huge_page(h); i++) {
page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
1 << PG_referenced | 1 << PG_dirty |
@@ -1313,13 +1374,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
set_page_refcounted(page);
if (hstate_is_gigantic(h)) {
/*
- * Temporarily drop the hugetlb_lock, because
- * we might block in free_gigantic_page().
+ * Temporarily drop the hugetlb_lock only when this type of
+ * HugeTLB page does not support vmemmap optimization (which
+ * contex do not hold the hugetlb_lock), because we might block
+ * in free_gigantic_page().
*/
- spin_unlock(&hugetlb_lock);
+ if (!free_vmemmap_pages_per_hpage(h))
+ spin_unlock(&hugetlb_lock);
destroy_compound_gigantic_page(page, huge_page_order(h));
free_gigantic_page(page, huge_page_order(h));
- spin_lock(&hugetlb_lock);
+ if (!free_vmemmap_pages_per_hpage(h))
+ spin_lock(&hugetlb_lock);
} else {
__free_pages(page, huge_page_order(h));
}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c464c5db8967..d080488cde16 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -197,18 +197,6 @@
(__boundary - 1 < (end) - 1) ? __boundary : (end); \
})

-/*
- * How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
- *
- * Todo: Now it is zero, because all infrastructure is not ready. Once all the
- * infrastructure is ready, we will rework this function to support the feature.
- */
-static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
-{
- return 0;
-}
-
static inline unsigned int vmemmap_pages_per_hpage(struct hstate *h)
{
return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 6923f03534d5..bf22cd003acb 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -12,9 +12,26 @@

#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+
+/*
+ * How many vmemmap pages associated with a HugeTLB page that can be freed
+ * to the buddy allocator.
+ *
+ * Todo: Now it is zero, because all infrastructure is not ready. Once all the
+ * infrastructure is ready, we will rework this function to support the feature.
+ */
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+ return 0;
+}
#else
static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
{
}
+
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+ return 0;
+}
#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
#endif /* _LINUX_HUGETLB_VMEMMAP_H */
--
2.11.0