linux-kernel - [PATCH 3/3] sparc32: mm: Only call ctor()/dtor() functions for first and last user

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Date:   Tue, 26 May 2020 18:33:02 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     sparclinux@...r.kernel.org, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "David S . Miller" <davem@...emloft.net>,
        Guenter Roeck <linux@...ck-us.net>,
        Mike Rapoport <rppt@...nel.org>, kernel-team@...roid.com
Subject: [PATCH 3/3] sparc32: mm: Only call ctor()/dtor() functions for first and last user

The SRMMU page-table allocator allocates multiple PTE tables per page,
since they are only 1K in size. However, this means that calls to
pgtable_pte_page_{ctor,dtor}() must be serialised and performed only by
the first and last page-table allocation for the page respectively.

Use the page reference count to track how many PTE tables we have
allocated for a given page returned by the SRMMU allocator and only
call the ctor()/dtor() functions for the first and last user respectively.

Cc: David S. Miller <davem@...emloft.net>
Fixes: 8c8f3156dd40 ("sparc32: mm: Reduce allocation size for PMD and PTE tables")
Signed-off-by: Will Deacon <will@...nel.org>
---
 arch/sparc/mm/srmmu.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/sparc/mm/srmmu.c b/arch/sparc/mm/srmmu.c
index 589370a21b12..116d19a390f2 100644
--- a/arch/sparc/mm/srmmu.c
+++ b/arch/sparc/mm/srmmu.c
@@ -364,9 +364,13 @@ pgtable_t pte_alloc_one(struct mm_struct *mm)
 	if ((ptep = pte_alloc_one_kernel(mm)) == 0)
 		return NULL;
 	page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT);
-	if (!pgtable_pte_page_ctor(page)) {
-		return NULL;
+	spin_lock(&mm->page_table_lock);
+	if (page_ref_inc_return(page) == 2 && !pgtable_pte_page_ctor(page)) {
+		page_ref_dec(page);
+		ptep = NULL;
 	}
+	spin_unlock(&mm->page_table_lock);
+
 	return ptep;
 }
 
@@ -375,7 +379,11 @@ void pte_free(struct mm_struct *mm, pgtable_t ptep)
 	struct page *page;
 
 	page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> PAGE_SHIFT);
-	pgtable_pte_page_dtor(page);
+	spin_lock(&mm->page_table_lock);
+	if (page_ref_dec_return(page) == 1)
+		pgtable_pte_page_dtor(page);
+	spin_unlock(&mm->page_table_lock);
+
 	srmmu_free_nocache(ptep, SRMMU_PTE_TABLE_SIZE);
 }
 
-- 
2.27.0.rc0.183.gde8f92d652-goog