[RFC] Calculate exactly how many ptr is needed for pgd

[RFC] Calculate exactly how many ptr is needed for pgd

[Songmao Tian]

Under 32-bit kernel with 4k page, a page is needed for a pgd,
but when page size > 4k, a page will be too much.

diff --git a/include/asm-mips/pgtable-32.h b/include/asm-mips/pgtable-32.h
index 2fbd47e..2a16240 100644
--- a/include/asm-mips/pgtable-32.h
+++ b/include/asm-mips/pgtable-32.h
@@ -67,7 +67,8 @@ extern int add_temporary_entry(unsigned long entrylo0, 
unsigned long entrylo1,
 #define PTE_ORDER    0
 #endif
 
-#define PTRS_PER_PGD    ((PAGE_SIZE << PGD_ORDER) / sizeof(pgd_t))
+/* Using a page for pgd will be a waste when page size > 4k */
+#define PTRS_PER_PGD    (1 << (32 - PGDIR_SHIFT))
 #define PTRS_PER_PTE    ((PAGE_SIZE << PTE_ORDER) / sizeof(pte_t))
 
 #define USER_PTRS_PER_PGD    (0x80000000UL/PGDIR_SIZE)

Fwd: [RFC] Calculate exactly how many ptr is needed for pgd

[Dajie Tan]

It works well. If I start xorg+xfce, it can save 2.5MB memory in my system.

Thanks, songmao.

2007/7/31, Songmao Tian <tiansm@lemote.com>:
> Under 32-bit kernel with 4k page, a page is needed for a pgd,
> but when page size > 4k, a page will be too much.
>
> diff --git a/include/asm-mips/pgtable-32.h b/include/asm-mips/pgtable-32.h
> index 2fbd47e..2a16240 100644
> --- a/include/asm-mips/pgtable-32.h
> +++ b/include/asm-mips/pgtable-32.h
> @@ -67,7 +67,8 @@ extern int add_temporary_entry(unsigned long entrylo0,
> unsigned long entrylo1,
>  #define PTE_ORDER    0
>  #endif
>
> -#define PTRS_PER_PGD    ((PAGE_SIZE << PGD_ORDER) / sizeof(pgd_t))
> +/* Using a page for pgd will be a waste when page size > 4k */
> +#define PTRS_PER_PGD    (1 << (32 - PGDIR_SHIFT))
>  #define PTRS_PER_PTE    ((PAGE_SIZE << PTE_ORDER) / sizeof(pte_t))
>
>  #define USER_PTRS_PER_PGD    (0x80000000UL/PGDIR_SIZE)
>
>


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