[PATCH -v2 06/10] m68k,mm: Improve kernel_page_table()

Peter Zijlstra
2020-01-31T12:56:13Z

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>
---
arch/m68k/include/asm/motorola_pgtable.h | 13 +++++++
arch/m68k/mm/init.c | 5 ---
arch/m68k/mm/motorola.c | 51 +++++++++++++++++--------------
3 files changed, 41 insertions(+), 28 deletions(-)

--- 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 - Desciptor 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)
--- 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(v
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
}

--- a/arch/m68k/mm/motorola.c
+++ b/arch/m68k/mm/motorola.c
@@ -72,27 +72,35 @@ extern __initdata unsigned long m68k_ini

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;
+
+ 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(pte_table);
+ mmu_page_ctor(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);
+ last_pte_table = pte_table;
+ }

- clear_page(ptablep);
- mmu_page_ctor(ptablep);
+ last_pte_table += PTRS_PER_PTE;

- return ptablep;
+ 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;

@@ -111,25 +119,25 @@ static pmd_t * __init kernel_ptr_table(v
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)
@@ -192,8 +200,7 @@ 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;