From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx>
---
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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