[PATCH v4 13/21] mm/hugetlb: Use PG_slab to indicate split pmd

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

When we allocate hugetlb page from buddy, we may need split huge pmd
to pte. When we free the hugetlb page, we can merge pte to pmd. So
we need to distinguish whether the previous pmd has been split. The
page table is not allocated from slab. So we can reuse the PG_slab
to indicate that the pmd has been split.

Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/hugetlb_vmemmap.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index ae9dbfb682ab..58bff13a2301 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -262,6 +262,25 @@ static void remap_huge_page_pmd_vmemmap(struct hstate *h, pmd_t *pmd,
 	flush_tlb_kernel_range(start, end);
 }
 
+static inline bool pmd_split(pmd_t *pmd)
+{
+	return PageSlab(pmd_page(*pmd));
+}
+
+static inline void set_pmd_split(pmd_t *pmd)
+{
+	/*
+	 * We should not use slab for page table allocation. So we can set
+	 * PG_slab to indicate that the pmd has been split.
+	 */
+	__SetPageSlab(pmd_page(*pmd));
+}
+
+static inline void clear_pmd_split(pmd_t *pmd)
+{
+	__ClearPageSlab(pmd_page(*pmd));
+}
+
 static void __remap_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
 					  unsigned long start,
 					  unsigned long end,
@@ -326,11 +345,12 @@ void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
 	ptl = vmemmap_pmd_lock(pmd);
 	remap_huge_page_pmd_vmemmap(h, pmd, (unsigned long)head, &remap_pages,
 				    __remap_huge_page_pte_vmemmap);
-	if (!freed_vmemmap_hpage_dec(pmd_page(*pmd))) {
+	if (!freed_vmemmap_hpage_dec(pmd_page(*pmd)) && pmd_split(pmd)) {
 		/*
 		 * Todo:
 		 * Merge pte to huge pmd if it has ever been split.
 		 */
+		clear_pmd_split(pmd);
 	}
 	spin_unlock(ptl);
 }
@@ -412,8 +432,10 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 	BUG_ON(!pmd);
 
 	ptl = vmemmap_pmd_lock(pmd);
-	if (vmemmap_pmd_huge(pmd))
+	if (vmemmap_pmd_huge(pmd)) {
 		split_vmemmap_huge_page(head, pmd);
+		set_pmd_split(pmd);
+	}
 
 	remap_huge_page_pmd_vmemmap(h, pmd, (unsigned long)head, &free_pages,
 				    __free_huge_page_pte_vmemmap);
-- 
2.11.0