linux-kernel - [PATCH] mm: Unify page_table

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Date:   Mon, 27 Nov 2017 09:59:51 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] mm: Unify page_table_lock allocation pattern

Subject: mm: Unify page_table_lock allocation pattern
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Nov 27 09:35:04 CET 2017

There are two different patterns wrt page_table_lock and allocating
new pages. Get rid of this diversity.

I picked this variant because it does less work under the lock and
makes page_table_lock a leaf lock.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 mm/memory.c |   33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4100,11 +4100,14 @@ int __p4d_alloc(struct mm_struct *mm, pg
 	smp_wmb(); /* See comment in __pte_alloc */
 
 	spin_lock(&mm->page_table_lock);
-	if (pgd_present(*pgd))		/* Another has populated it */
-		p4d_free(mm, new);
-	else
+	if (!pgd_present(*pgd)) {
 		pgd_populate(mm, pgd, new);
+		new = NULL;
+	}
 	spin_unlock(&mm->page_table_lock);
+	if (new)
+		p4d_free(mm, new);
+
 	return 0;
 }
 #endif /* __PAGETABLE_P4D_FOLDED */
@@ -4124,17 +4127,19 @@ int __pud_alloc(struct mm_struct *mm, p4
 
 	spin_lock(&mm->page_table_lock);
 #ifndef __ARCH_HAS_5LEVEL_HACK
-	if (p4d_present(*p4d))		/* Another has populated it */
-		pud_free(mm, new);
-	else
+	if (!p4d_present(*p4d)) {
 		p4d_populate(mm, p4d, new);
+		new = NULL;
+	}
 #else
-	if (pgd_present(*p4d))		/* Another has populated it */
-		pud_free(mm, new);
-	else
+	if (!pgd_present(*p4d)) {
 		pgd_populate(mm, p4d, new);
+		new = NULL;
+	}
 #endif /* __ARCH_HAS_5LEVEL_HACK */
 	spin_unlock(&mm->page_table_lock);
+	if (new)
+		pud_free(mm, new);
 	return 0;
 }
 #endif /* __PAGETABLE_PUD_FOLDED */
@@ -4158,16 +4163,18 @@ int __pmd_alloc(struct mm_struct *mm, pu
 	if (!pud_present(*pud)) {
 		mm_inc_nr_pmds(mm);
 		pud_populate(mm, pud, new);
-	} else	/* Another has populated it */
-		pmd_free(mm, new);
+		new = NULL;
+	}
 #else
 	if (!pgd_present(*pud)) {
 		mm_inc_nr_pmds(mm);
 		pgd_populate(mm, pud, new);
-	} else /* Another has populated it */
-		pmd_free(mm, new);
+		new = NULL;
+	}
 #endif /* __ARCH_HAS_4LEVEL_HACK */
 	spin_unlock(ptl);
+	if (new)
+		pmd_free(mm, new);
 	return 0;
 }
 #endif /* __PAGETABLE_PMD_FOLDED */