[PATCH v6 03/12] thp: copy_huge_pmd(): copy huge zero page

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

It's easy to copy huge zero page. Just set destination pmd to huge zero
page.

It's safe to copy huge zero page since we have none yet :-p

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 mm/huge_memory.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index fc2cada..183127c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -701,6 +701,18 @@ static inline struct page *alloc_hugepage(int defrag)
 }
 #endif
 
+static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
+		struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd)
+{
+	pmd_t entry;
+	entry = pfn_pmd(huge_zero_pfn, vma->vm_page_prot);
+	entry = pmd_wrprotect(entry);
+	entry = pmd_mkhuge(entry);
+	set_pmd_at(mm, haddr, pmd, entry);
+	pgtable_trans_huge_deposit(mm, pgtable);
+	mm->nr_ptes++;
+}
+
 int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			       unsigned long address, pmd_t *pmd,
 			       unsigned int flags)
@@ -778,6 +790,16 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		pte_free(dst_mm, pgtable);
 		goto out_unlock;
 	}
+	/*
+	 * mm->pagetable lock is enough to be sure that huge zero pmd is not
+	 * under splitting since we don't split the page itself, only pmd to
+	 * a page table.
+	 */
+	if (is_huge_zero_pmd(pmd)) {
+		set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd);
+		ret = 0;
+		goto out_unlock;
+	}
 	if (unlikely(pmd_trans_splitting(pmd))) {
 		/* split huge page running from under us */
 		spin_unlock(&src_mm->page_table_lock);
-- 
1.7.11.7

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