[RFC PATCH 06/24] mm/hugetlb: Introduce pgtable allocation/freeing helpers

[Muchun Song <songmuchun@bytedance.com>]

On some architectures, the vmemmap areas use huge page mapping.
If we want to free the unused vmemmap pages, we have to split
the huge pmd firstly. So we should pre-allocate pgtable to split
huge pmd.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/hugetlb.h |  17 ++++++
 mm/hugetlb.c            | 117 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index eed3dd3bd626..ace304a6196c 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -593,6 +593,23 @@ static inline unsigned int blocks_per_huge_page(struct hstate *h)
 
 #include <asm/hugetlb.h>
 
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+#ifndef arch_vmemmap_support_huge_mapping
+static inline bool arch_vmemmap_support_huge_mapping(void)
+{
+	return false;
+}
+#endif
+
+#ifndef VMEMMAP_HPAGE_SHIFT
+#define VMEMMAP_HPAGE_SHIFT		PMD_SHIFT
+#endif
+#define VMEMMAP_HPAGE_ORDER		(VMEMMAP_HPAGE_SHIFT - PAGE_SHIFT)
+#define VMEMMAP_HPAGE_NR		(1 << VMEMMAP_HPAGE_ORDER)
+#define VMEMMAP_HPAGE_SIZE		((1UL) << VMEMMAP_HPAGE_SHIFT)
+#define VMEMMAP_HPAGE_MASK		(~(VMEMMAP_HPAGE_SIZE - 1))
+#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
+
 #ifndef is_hugepage_only_range
 static inline int is_hugepage_only_range(struct mm_struct *mm,
 					unsigned long addr, unsigned long len)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f1b2b733b49b..d6ae9b6876be 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1295,11 +1295,108 @@ static inline void destroy_compound_gigantic_page(struct page *page,
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 #define RESERVE_VMEMMAP_NR	2U
 
+#define page_huge_pte(page)	((page)->pmd_huge_pte)
+
 static inline unsigned int nr_free_vmemmap(struct hstate *h)
 {
 	return h->nr_free_vmemmap_pages;
 }
 
+static inline unsigned int nr_vmemmap(struct hstate *h)
+{
+	return nr_free_vmemmap(h) + RESERVE_VMEMMAP_NR;
+}
+
+static inline unsigned long nr_vmemmap_size(struct hstate *h)
+{
+	return (unsigned long)nr_vmemmap(h) << PAGE_SHIFT;
+}
+
+static inline unsigned int nr_pgtable(struct hstate *h)
+{
+	unsigned long vmemmap_size = nr_vmemmap_size(h);
+
+	if (!arch_vmemmap_support_huge_mapping())
+		return 0;
+
+	/*
+	 * No need pre-allocate page tabels when there is no vmemmap pages
+	 * to free.
+	 */
+	if (!nr_free_vmemmap(h))
+		return 0;
+
+	return ALIGN(vmemmap_size, VMEMMAP_HPAGE_SIZE) >> VMEMMAP_HPAGE_SHIFT;
+}
+
+static inline void vmemmap_pgtable_init(struct page *page)
+{
+	page_huge_pte(page) = NULL;
+}
+
+static void vmemmap_pgtable_deposit(struct page *page, pte_t *pte_p)
+{
+	pgtable_t pgtable = virt_to_page(pte_p);
+
+	/* FIFO */
+	if (!page_huge_pte(page))
+		INIT_LIST_HEAD(&pgtable->lru);
+	else
+		list_add(&pgtable->lru, &page_huge_pte(page)->lru);
+	page_huge_pte(page) = pgtable;
+}
+
+static pte_t *vmemmap_pgtable_withdraw(struct page *page)
+{
+	pgtable_t pgtable;
+
+	/* FIFO */
+	pgtable = page_huge_pte(page);
+	if (unlikely(!pgtable))
+		return NULL;
+	page_huge_pte(page) = list_first_entry_or_null(&pgtable->lru,
+						       struct page, lru);
+	if (page_huge_pte(page))
+		list_del(&pgtable->lru);
+	return page_to_virt(pgtable);
+}
+
+static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
+{
+	int i;
+	pte_t *pte_p;
+	unsigned int nr = nr_pgtable(h);
+
+	if (!nr)
+		return 0;
+
+	vmemmap_pgtable_init(page);
+
+	for (i = 0; i < nr; i++) {
+		pte_p = pte_alloc_one_kernel(&init_mm);
+		if (!pte_p)
+			goto out;
+		vmemmap_pgtable_deposit(page, pte_p);
+	}
+
+	return 0;
+out:
+	while (i-- && (pte_p = vmemmap_pgtable_withdraw(page)))
+		pte_free_kernel(&init_mm, pte_p);
+	return -ENOMEM;
+}
+
+static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
+{
+	pte_t *pte_p;
+
+	if (!nr_pgtable(h))
+		return;
+
+	while ((pte_p = vmemmap_pgtable_withdraw(page)))
+		pte_free_kernel(&init_mm, pte_p);
+}
+
 static void __init hugetlb_vmemmap_init(struct hstate *h)
 {
 	unsigned int order = huge_page_order(h);
@@ -1323,6 +1420,15 @@ static void __init hugetlb_vmemmap_init(struct hstate *h)
 static inline void hugetlb_vmemmap_init(struct hstate *h)
 {
 }
+
+static inline int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
+{
+	return 0;
+}
+
+static inline void vmemmap_pgtable_free(struct hstate *h, struct page *page)
+{
+}
 #endif
 
 static void update_and_free_page(struct hstate *h, struct page *page)
@@ -1531,6 +1637,9 @@ void free_huge_page(struct page *page)
 
 static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
 {
+	/* Must be called before the initialization of @page->lru */
+	vmemmap_pgtable_free(h, page);
+
 	INIT_LIST_HEAD(&page->lru);
 	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
 	set_hugetlb_cgroup(page, NULL);
@@ -1783,6 +1892,14 @@ static struct page *alloc_fresh_huge_page(struct hstate *h,
 	if (!page)
 		return NULL;
 
+	if (vmemmap_pgtable_prealloc(h, page)) {
+		if (hstate_is_gigantic(h))
+			free_gigantic_page(page, huge_page_order(h));
+		else
+			put_page(page);
+		return NULL;
+	}
+
 	if (hstate_is_gigantic(h))
 		prep_compound_gigantic_page(page, huge_page_order(h));
 	prep_new_huge_page(h, page, page_to_nid(page));
-- 
2.20.1