linux-kernel - [PATCH v2 2/5] mm/cow: don't bother write protecting already write-protected huge pages

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Message-Id: <20181016131343.20556-3-npiggin@gmail.com>
Date:   Tue, 16 Oct 2018 23:13:40 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Ley Foon Tan <ley.foon.tan@...el.com>
Subject: [PATCH v2 2/5] mm/cow: don't bother write protecting already write-protected huge pages

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

Signed-off-by: Nicholas Piggin <npiggin@...il.com>
---
 mm/huge_memory.c | 14 ++++++++++----
 mm/hugetlb.c     |  2 +-
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 58269f8ba7c4..0fb0e3025f98 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;
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5c390f5a5207..54a4dcb6ee21 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3287,7 +3287,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 			}
 			set_huge_swap_pte_at(dst, addr, dst_pte, entry, sz);
 		} else {
-			if (cow) {
+			if (cow && huge_pte_write(entry)) {
 				/*
 				 * No need to notify as we are downgrading page
 				 * table protection not changing it to point
-- 
2.18.0