[PATCH] mm: Unify page_table_lock allocation pattern

[Peter Zijlstra]

Subject: mm: Unify page_table_lock allocation pattern
From: Peter Zijlstra <pet...@infradead.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) <pet...@infradead.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 */