[v2,1/2] ARM: mm: fix no-MMU ZERO_PAGE() implementation
Commit Message
Actually in no-MMU SoCs(i.e. i.MXRT) ZERO_PAGE(vaddr) expands to
```
virt_to_page(0)
```
that in order expands to:
```
pfn_to_page(virt_to_pfn(0))
```
and then virt_to_pfn(0) to:
```
#define virt_to_pfn(0) \
((((unsigned long)(0) - PAGE_OFFSET) >> PAGE_SHIFT) + \
PHYS_PFN_OFFSET)
```
where PAGE_OFFSET and PHYS_PFN_OFFSET are the DRAM offset(0x80000000) and
PAGE_SHIFT is 12. This way we obtain 16MB(0x01000000) summed to the base of
DRAM(0x80000000).
When ZERO_PAGE(0) is then used, for example in bio_add_page(), the page
gets an address that is out of DRAM bounds.
So instead of using fake virtual page 0 let's allocate a dedicated
zero_page during paging_init() and assign it to a global 'struct page *
empty_zero_page' the same way mmu.c does and it's the same approach used
in m68k with commit dc068f462179 as discussed here[0]. Then let's move
ZERO_PAGE() definition to the top of pgtable.h to be in common between
mmu.c and nommu.c.
[0]: https://lore.kernel.org/linux-m68k/2a462b23-5b8e-bbf4-ec7d-778434a3b9d7@google.com/T/#m1266ceb63ad140743174d6b3070364d3c9a5179b
Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
---
V1->V2:
* improve commit log as suggested by Arnd Bergmann
---
arch/arm/include/asm/pgtable-nommu.h | 6 ------
arch/arm/include/asm/pgtable.h | 16 +++++++++-------
arch/arm/mm/nommu.c | 19 +++++++++++++++++++
3 files changed, 28 insertions(+), 13 deletions(-)
Comments
Hello Arnd, Russell, All,
is this patch ok or has it some changes to do?
While instead [PATCH 2/2] has a NAK and can be dropped.
Best regards
On Wed, Oct 19, 2022, at 00:25, Giulio Benetti wrote:
> Actually in no-MMU SoCs(i.e. i.MXRT) ZERO_PAGE(vaddr) expands to
> ```
> virt_to_page(0)
> ```
> that in order expands to:
> ```
> pfn_to_page(virt_to_pfn(0))
> ```
> and then virt_to_pfn(0) to:
> ```
> #define virt_to_pfn(0) \
> ((((unsigned long)(0) - PAGE_OFFSET) >> PAGE_SHIFT) + \
> PHYS_PFN_OFFSET)
> ```
> where PAGE_OFFSET and PHYS_PFN_OFFSET are the DRAM offset(0x80000000) and
> PAGE_SHIFT is 12. This way we obtain 16MB(0x01000000) summed to the base of
> DRAM(0x80000000).
> When ZERO_PAGE(0) is then used, for example in bio_add_page(), the page
> gets an address that is out of DRAM bounds.
> So instead of using fake virtual page 0 let's allocate a dedicated
> zero_page during paging_init() and assign it to a global 'struct page *
> empty_zero_page' the same way mmu.c does and it's the same approach used
> in m68k with commit dc068f462179 as discussed here[0]. Then let's move
> ZERO_PAGE() definition to the top of pgtable.h to be in common between
> mmu.c and nommu.c.
>
> [0]:
> https://lore.kernel.org/linux-m68k/2a462b23-5b8e-bbf4-ec7d-778434a3b9d7@google.com/T/#m1266ceb63ad140743174d6b3070364d3c9a5179b
>
> Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
On Fri, Nov 4, 2022, at 21:07, Giulio Benetti wrote:
> Hello Arnd, Russell, All,
>
> is this patch ok or has it some changes to do?
Looks ok to me, please add it to Russell's patch
tracker at:
https://www.arm.linux.org.uk/developer/patches/
The patch description could be improved a little ("Link:"
tag for the URL, avoiding the wikitext markup, etc), but
it's more important to actually get the bug fixed.
Arnd
Hi Arnd,
On 04/11/22 21:28, Arnd Bergmann wrote:
> On Fri, Nov 4, 2022, at 21:07, Giulio Benetti wrote:
>> Hello Arnd, Russell, All,
>>
>> is this patch ok or has it some changes to do?
>
> Looks ok to me, please add it to Russell's patch
> tracker at:
> https://www.arm.linux.org.uk/developer/patches/
I've submitted it with your Reviewed-by:
https://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=9266/1
> The patch description could be improved a little ("Link:"
> tag for the URL, avoiding the wikitext markup, etc),
Oh, I was not aware of "Link:" and avoiding wikitext markup. Next
time I won't use it and I will use Link instead. I'll grep some other
commit log to find out a correct way.
> but
> it's more important to actually get the bug fixed.
Thank you for guiding me
Best regards
@@ -44,12 +44,6 @@
typedef pte_t *pte_addr_t;
-/*
- * ZERO_PAGE is a global shared page that is always zero: used
- * for zero-mapped memory areas etc..
- */
-#define ZERO_PAGE(vaddr) (virt_to_page(0))
-
/*
* Mark the prot value as uncacheable and unbufferable.
*/
@@ -10,6 +10,15 @@
#include <linux/const.h>
#include <asm/proc-fns.h>
+#ifndef __ASSEMBLY__
+/*
+ * 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)
+#endif
+
#ifndef CONFIG_MMU
#include <asm-generic/pgtable-nopud.h>
@@ -139,13 +148,6 @@ extern pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
*/
#ifndef __ASSEMBLY__
-/*
- * 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 pgd_t swapper_pg_dir[PTRS_PER_PGD];
@@ -26,6 +26,13 @@
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;
+EXPORT_SYMBOL(empty_zero_page);
+
#ifdef CONFIG_ARM_MPU
struct mpu_rgn_info mpu_rgn_info;
#endif
@@ -148,9 +155,21 @@ 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);
}
/*