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ARM: 9266/1: mm: fix no-MMU ZERO_PAGE() implementation

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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:
```
        ((((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 dc068f4621 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/#m1266ceb63
ad140743174d6b3070364d3c9a5179b

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Giulio Benetti 2022-11-04 21:46:18 +01:00 • committed by Russell King (Oracle)
parent 612695bccf
commit 340a982825
3 changed files with 28 additions and 13 deletions
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6
arch/arm/include/asm/pgtable-nommu.h
View file

@ -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.
*/

16
arch/arm/include/asm/pgtable.h
View file

@ -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];

19
arch/arm/mm/nommu.c
View file

@ -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);
}
/*