m68k: mm: Improve kernel_page_table()

With the PTE-tables now only being 256 bytes, allocating a full page
for them is a giant waste. Start by improving the boot time allocator
such that init_mm initialization will at least have optimal memory
density.

Much thanks to Will Deacon in help with debugging and ferreting out
lost information on these dusty MMUs.

Notes:

 - _TABLE_MASK is reduced to account for the shorter (256 byte)
   alignment of pte-tables, per the manual, table entries should only
   ever have state in the low 4 bits (Used,WrProt,Desc1,Desc0) so it is
   still longer than strictly required. (Thanks Will!!!)

 - Also use kernel_page_table() for the 020/030 zero_pgtable case and
   consequently remove the zero_pgtable init hack (will fix up later).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200131125403.768263973@infradead.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

3 files changed, 41 insertions, 28 deletions

diff --git a/arch/m68k/include/asm/motorola_pgtable.h b/arch/m68k/include/asm/motorola_pgtable.h
index 4d94e462bb2b..2ad0f4166841 100644
--- a/arch/m68k/include/asm/motorola_pgtable.h
+++ b/arch/m68k/include/asm/motorola_pgtable.h
@@ -23,7 +23,18 @@
 #define _DESCTYPE_MASK	0x003
 
 #define _CACHEMASK040	(~0x060)
-#define _TABLE_MASK	(0xfffffe00)
+
+/*
+ * Currently set to the minimum alignment of table pointers (256 bytes).
+ * The hardware only uses the low 4 bits for state:
+ *
+ *    3 - Used
+ *    2 - Write Protected
+ *  0,1 - Descriptor Type
+ *
+ * and has the rest of the bits reserved.
+ */
+#define _TABLE_MASK	(0xffffff00)
 
 #define _PAGE_TABLE	(_PAGE_SHORT)
 #define _PAGE_CHG_MASK  (PAGE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY | _PAGE_NOCACHE)
diff --git a/arch/m68k/mm/init.c b/arch/m68k/mm/init.c
index 27c453f4fffe..a9b6d26cff8a 100644
--- a/arch/m68k/mm/init.c
+++ b/arch/m68k/mm/init.c
@@ -42,7 +42,6 @@ EXPORT_SYMBOL(empty_zero_page);
 
 #if !defined(CONFIG_SUN3) && !defined(CONFIG_COLDFIRE)
 extern void init_pointer_table(unsigned long ptable);
-extern pmd_t *zero_pgtable;
 #endif
 
 #ifdef CONFIG_MMU
@@ -135,10 +134,6 @@ static inline void init_pointer_tables(void)
 		if (pud_present(*pud))
 			init_pointer_table(pgd_page_vaddr(kernel_pg_dir[i]));
 	}
-
-	/* insert also pointer table that we used to unmap the zero page */
-	if (zero_pgtable)
-		init_pointer_table((unsigned long)zero_pgtable);
 #endif
 }
 
diff --git a/arch/m68k/mm/motorola.c b/arch/m68k/mm/motorola.c
index c888ef46da3e..600f9c1d96f8 100644
--- a/arch/m68k/mm/motorola.c
+++ b/arch/m68k/mm/motorola.c
@@ -174,27 +174,35 @@ extern __initdata unsigned long m68k_init_mapped_size;
 
 extern unsigned long availmem;
 
+static pte_t *last_pte_table __initdata = NULL;
+
 static pte_t * __init kernel_page_table(void)
 {
-	pte_t *ptablep;
+	pte_t *pte_table = last_pte_table;
 
-	ptablep = (pte_t *)memblock_alloc_low(PAGE_SIZE, PAGE_SIZE);
-	if (!ptablep)
-		panic("%s: Failed to allocate %lu bytes align=%lx\n",
-		      __func__, PAGE_SIZE, PAGE_SIZE);
+	if (((unsigned long)last_pte_table & ~PAGE_MASK) == 0) {
+		pte_table = (pte_t *)memblock_alloc_low(PAGE_SIZE, PAGE_SIZE);
+		if (!pte_table) {
+			panic("%s: Failed to allocate %lu bytes align=%lx\n",
+					__func__, PAGE_SIZE, PAGE_SIZE);
+		}
 
-	clear_page(ptablep);
-	mmu_page_ctor(ptablep);
+		clear_page(pte_table);
+		mmu_page_ctor(pte_table);
 
-	return ptablep;
+		last_pte_table = pte_table;
+	}
+
+	last_pte_table += PTRS_PER_PTE;
+
+	return pte_table;
 }
 
-static pmd_t *last_pgtable __initdata = NULL;
-pmd_t *zero_pgtable __initdata = NULL;
+static pmd_t *last_pmd_table __initdata = NULL;
 
 static pmd_t * __init kernel_ptr_table(void)
 {
-	if (!last_pgtable) {
+	if (!last_pmd_table) {
 		unsigned long pmd, last;
 		int i;
 
@@ -213,25 +221,25 @@ static pmd_t * __init kernel_ptr_table(void)
 				last = pmd;
 		}
 
-		last_pgtable = (pmd_t *)last;
+		last_pmd_table = (pmd_t *)last;
 #ifdef DEBUG
-		printk("kernel_ptr_init: %p\n", last_pgtable);
+		printk("kernel_ptr_init: %p\n", last_pmd_table);
 #endif
 	}
 
-	last_pgtable += PTRS_PER_PMD;
-	if (((unsigned long)last_pgtable & ~PAGE_MASK) == 0) {
-		last_pgtable = (pmd_t *)memblock_alloc_low(PAGE_SIZE,
+	last_pmd_table += PTRS_PER_PMD;
+	if (((unsigned long)last_pmd_table & ~PAGE_MASK) == 0) {
+		last_pmd_table = (pmd_t *)memblock_alloc_low(PAGE_SIZE,
 							   PAGE_SIZE);
-		if (!last_pgtable)
+		if (!last_pmd_table)
 			panic("%s: Failed to allocate %lu bytes align=%lx\n",
 			      __func__, PAGE_SIZE, PAGE_SIZE);
 
-		clear_page(last_pgtable);
-		mmu_page_ctor(last_pgtable);
+		clear_page(last_pmd_table);
+		mmu_page_ctor(last_pmd_table);
 	}
 
-	return last_pgtable;
+	return last_pmd_table;
 }
 
 static void __init map_node(int node)
@@ -294,8 +302,7 @@ static void __init map_node(int node)
 #ifdef DEBUG
 				printk ("[zero map]");
 #endif
-				zero_pgtable = kernel_ptr_table();
-				pte_dir = (pte_t *)zero_pgtable;
+				pte_dir = kernel_page_table();
 				pmd_set(pmd_dir, pte_dir);
 
 				pte_val(*pte_dir++) = 0;