diff mbox series

[v2,2/2] ARM: mm: convert empty_zero_page to array for consistency

Message ID 20221018222503.90118-2-giulio.benetti@benettiengineering.com
State New
Headers
Series [v2,1/2] ARM: mm: fix no-MMU ZERO_PAGE() implementation |

Commit Message

Giulio Benetti Oct. 18, 2022, 10:25 p.m. UTC 
  ARM architecture is the only one to have empty_zero_page to be a
struct page pointer, while in all other implementations empty_zero_page is
a data pointer or directly an array(the zero page itself). So let's convert
empty_zero_page to an array for consistency and to avoid an early
allocation+dcache flush. Being the array in .bss it will be cleared earlier
in a more linear way(and a bit faster) way.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
---
V1->V2:
* create patch suggested by Arnd Bergmann
---
 arch/arm/include/asm/pgtable.h |  4 ++--
 arch/arm/mm/mmu.c              | 10 +---------
 arch/arm/mm/nommu.c            | 14 +-------------
 3 files changed, 4 insertions(+), 24 deletions(-)

Comments

Russell King (Oracle) Oct. 19, 2022, 2:44 p.m. UTC | #1 
On Wed, Oct 19, 2022 at 12:25:03AM +0200, Giulio Benetti wrote:
> ARM architecture is the only one to have empty_zero_page to be a
> struct page pointer, while in all other implementations empty_zero_page is
> a data pointer or directly an array(the zero page itself). So let's convert
> empty_zero_page to an array for consistency and to avoid an early
> allocation+dcache flush. Being the array in .bss it will be cleared earlier
> in a more linear way(and a bit faster) way.
> 
> Suggested-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>

I'm completely against this approach. It introduces inefficiencies in
paths we don't need, and also means that the zero page is at a fixed
location relative to the kernel, neither of which I like in the
slightest.

Thanks.
Giulio Benetti Oct. 19, 2022, 4:29 p.m. UTC | #2 
Hello Russell,

On 19/10/22 16:44, Russell King (Oracle) wrote:
> On Wed, Oct 19, 2022 at 12:25:03AM +0200, Giulio Benetti wrote:
>> ARM architecture is the only one to have empty_zero_page to be a
>> struct page pointer, while in all other implementations empty_zero_page is
>> a data pointer or directly an array(the zero page itself). So let's convert
>> empty_zero_page to an array for consistency and to avoid an early
>> allocation+dcache flush. Being the array in .bss it will be cleared earlier
>> in a more linear way(and a bit faster) way.
>>
>> Suggested-by: Arnd Bergmann <arnd@arndb.de>
>> Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
> 
> I'm completely against this approach. It introduces inefficiencies in
> paths we don't need, and also means that the zero page is at a fixed
> location relative to the kernel, neither of which I like in the
> slightest.

I haven't considered those details, I'm pretty new in this topic.
I was thinking with a no-mmu approach in my mind, that's why the
.bss approach. And also the exposure of the entire array to the other
subsystem is not a good idea.

Thank you for pointing me

Best regads
diff mbox series

Patch

diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
index ef48a55e9af8..de402b345f55 100644
--- a/arch/arm/include/asm/pgtable.h
+++ b/arch/arm/include/asm/pgtable.h
@@ -15,8 +15,8 @@ 
  * ZERO_PAGE is a global shared page that is always zero: used
  * for zero-mapped memory areas etc..
  */
-extern struct page *empty_zero_page;
-#define ZERO_PAGE(vaddr)	(empty_zero_page)
+extern unsigned long empty_zero_page[];
+#define ZERO_PAGE(vaddr)	(virt_to_page(empty_zero_page))
 #endif
 
 #ifndef CONFIG_MMU
diff --git a/arch/arm/mm/mmu.c b/arch/arm/mm/mmu.c
index 463fc2a8448f..f05a5471a45a 100644
--- a/arch/arm/mm/mmu.c
+++ b/arch/arm/mm/mmu.c
@@ -45,7 +45,7 @@  extern unsigned long __atags_pointer;
  * empty_zero_page is a special page that is used for
  * zero-initialized data and COW.
  */
-struct page *empty_zero_page;
+unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
 EXPORT_SYMBOL(empty_zero_page);
 
 /*
@@ -1760,8 +1760,6 @@  static void __init early_fixmap_shutdown(void)
  */
 void __init paging_init(const struct machine_desc *mdesc)
 {
-	void *zero_page;
-
 	pr_debug("physical kernel sections: 0x%08llx-0x%08llx\n",
 		 kernel_sec_start, kernel_sec_end);
 
@@ -1782,13 +1780,7 @@  void __init paging_init(const struct machine_desc *mdesc)
 
 	top_pmd = pmd_off_k(0xffff0000);
 
-	/* allocate the zero page. */
-	zero_page = early_alloc(PAGE_SIZE);
-
 	bootmem_init();
-
-	empty_zero_page = virt_to_page(zero_page);
-	__flush_dcache_page(NULL, empty_zero_page);
 }
 
 void __init early_mm_init(const struct machine_desc *mdesc)
diff --git a/arch/arm/mm/nommu.c b/arch/arm/mm/nommu.c
index c1494a4dee25..e0c3f59d1c5a 100644
--- a/arch/arm/mm/nommu.c
+++ b/arch/arm/mm/nommu.c
@@ -30,7 +30,7 @@  unsigned long vectors_base;
  * empty_zero_page is a special page that is used for
  * zero-initialized data and COW.
  */
-struct page *empty_zero_page;
+unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
 EXPORT_SYMBOL(empty_zero_page);
 
 #ifdef CONFIG_ARM_MPU
@@ -155,21 +155,9 @@  void __init adjust_lowmem_bounds(void)
  */
 void __init paging_init(const struct machine_desc *mdesc)
 {
-	void *zero_page;
-
 	early_trap_init((void *)vectors_base);
 	mpu_setup();
-
-	/* allocate the zero page. */
-	zero_page = memblock_alloc(PAGE_SIZE, PAGE_SIZE);
-	if (!zero_page)
-		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
-		      __func__, PAGE_SIZE, PAGE_SIZE);
-
 	bootmem_init();
-
-	empty_zero_page = virt_to_page(zero_page);
-	flush_dcache_page(empty_zero_page);
 }
 
 /*