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@davemloft.net>
Fixes: 8c8f3156 ("sparc32: mm: Reduce allocation size for PMD and PTE tables")
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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);
+	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);
 }