[PATCH 1/3] mm/cow: don't bother write protectig already write-protected huge pages

[Nicholas Piggin <npiggin@gmail.com>]

This is the THP equivalent for 1b2de5d039c8 ("mm/cow: don't bother write
protecting already write-protected pages").

Explicit hugetlb pages don't get the same treatment because they don't
appear to have the right accessor functions.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 mm/huge_memory.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9592cbd8530a..d9bae12978ef 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -973,8 +973,11 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 	mm_inc_nr_ptes(dst_mm);
 	pgtable_trans_huge_deposit(dst_mm, dst_pmd, pgtable);
 
-	pmdp_set_wrprotect(src_mm, addr, src_pmd);
-	pmd = pmd_mkold(pmd_wrprotect(pmd));
+	if (pmd_write(pmd)) {
+		pmdp_set_wrprotect(src_mm, addr, src_pmd);
+		pmd = pmd_wrprotect(pmd);
+	}
+	pmd = pmd_mkold(pmd);
 	set_pmd_at(dst_mm, addr, dst_pmd, pmd);
 
 	ret = 0;
@@ -1064,8 +1067,11 @@ int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		/* No huge zero pud yet */
 	}
 
-	pudp_set_wrprotect(src_mm, addr, src_pud);
-	pud = pud_mkold(pud_wrprotect(pud));
+	if (pud_write(pud)) {
+		pudp_set_wrprotect(src_mm, addr, src_pud);
+		pud = pud_wrprotect(pud);
+	}
+	pud = pud_mkold(pud);
 	set_pud_at(dst_mm, addr, dst_pud, pud);
 
 	ret = 0;
-- 
2.18.0