Re: 2G memory split

[Coywolf Qi Hunt <coywolf@gmail.com>]

2006/1/10, Jens Axboe <axboe@suse.de>:
> On Tue, Jan 10 2006, Byron Stanoszek wrote:
> > On Tue, 10 Jan 2006, Jens Axboe wrote:
> >
> > >>yes, i made it totally configurable in 2.4 days: 1:3, 2/2 and 3:1 splits
> > >>were possible. It was a larger patch to enable all this across x86, but
> > >>the Kconfig portion was removed a bit later because people _frequently_
> > >>misconfigured their kernels and then complained about the results.
> > >
> > >How is this different than all other sorts of misconfigurations? As far
> > >as I can tell, the biggest "problem" for some is if they depend on some
> > >binary module that will of course break with a different page offset.
> > >
> > >For simplicity, I didn't add more than the 2/2 split, where we could add
> > >even a 3/1 kernel/user or a 0.5/3.5 (I think sles8 had this).
> >
> > I prefer setting __PAGE_OFFSET to (0x78000000) on machines with 2GB of RAM.
> > This seems to let the kernel use the full 2GB of memory, rather than just
> > 1920-1984 MB (at least back in 2.4 days).
>
> A newer version, trying to cater to the various comments in here.
> Changes:
>
> - Add 1G_OPT split, meant for 1GiB machines. Uses 0xB0000000
> - Add 1G/3G split
> - Move the 2G/2G a little, so the full 2GiB of ram can be mapped.
> - Improve help text (I hope :)
> - Make option depend on EXPERIMENTAL.
> - Make the page.h a lot more readable.
>
> ---
>
> Add option for configuring the page offset, to better optimize the
> kernel for higher memory machines. Enables users to get rid of high
> memory for eg a 1GiB machine.
>
> Signed-off-by: Jens Axboe <axboe@suse.de>
>
> diff --git a/arch/i386/Kconfig b/arch/i386/Kconfig
> index d849c68..fcad8f7 100644
> --- a/arch/i386/Kconfig
> +++ b/arch/i386/Kconfig
> @@ -444,6 +464,32 @@ config HIGHMEM64G
>
>  endchoice
>
> +choice
> +       depends on NOHIGHMEM && EXPERIMENTAL
> +       prompt "Memory split"
> +       default DEFAULT_3G
> +       help
> +         Select the wanted split between kernel and user memory.
> +
> +         If the address range available to the kernel is less than the
> +         physical memory installed, the remaining memory will be available
> +         as "high memory". Accessing high memory is a little more costly
> +         than low memory, as it needs to be mapped into the kernel first.
> +
> +         Note that selecting anything but the default 3G/1G split will make
> +         your kernel incompatible with binary only modules.
> +
> +       config DEFAULT_3G
> +               bool "3G/1G user/kernel split"
> +       config DEFAULT_3G_OPT
> +               bool "3G/1G user/kernel split (for full 1G low memory)"
> +       config DEFAULT_2G
> +               bool "2G/2G user/kernel split"
> +       config DEFAULT_1G
> +               bool "1G/3G user/kernel split"

I don't like these names. Can't your invent better ones?
Having multiple defaults seems odd. See these maybe:

MEMSPLIT_U3_K1
MEMSPLIT_U11_K5
MEMSPLIT_U39_K44
MEMSPLIT_U1_K3

odd too? midnight here. |-)

> +
> +endchoice
> +
>  config HIGHMEM
>         bool
>         depends on HIGHMEM64G || HIGHMEM4G
> diff --git a/include/asm-i386/page.h b/include/asm-i386/page.h
> index 73296d9..7da50a1 100644
> --- a/include/asm-i386/page.h
> +++ b/include/asm-i386/page.h
> @@ -109,11 +109,23 @@ extern int page_is_ram(unsigned long pag
>
>  #endif /* __ASSEMBLY__ */
>
> +#if defined(CONFIG_DEFAULT_3G)
> +#define __PAGE_OFFSET_RAW      (0xC0000000)
> +#elif defined(CONFIG_DEFAULT_3G_OPT)
> +#define        __PAGE_OFFSET_RAW       (0xB0000000)
> +#elif defined(CONFIG_DEFAULT_2G)
> +#define __PAGE_OFFSET_RAW      (0x78000000)
> +#elif defined(CONFIG_DEFAULT_1G)
> +#define __PAGE_OFFSET_RAW      (0x40000000)
> +#else
> +#error "Bad user/kernel offset"
> +#endif
> +
>  #ifdef __ASSEMBLY__
> -#define __PAGE_OFFSET          (0xC0000000)
> +#define __PAGE_OFFSET          __PAGE_OFFSET_RAW
>  #define __PHYSICAL_START       CONFIG_PHYSICAL_START
>  #else
> -#define __PAGE_OFFSET          (0xC0000000UL)
> +#define __PAGE_OFFSET          ((unsigned long)__PAGE_OFFSET_RAW)
>  #define __PHYSICAL_START       ((unsigned long)CONFIG_PHYSICAL_START)
>  #endif
>  #define __KERNEL_START         (__PAGE_OFFSET + __PHYSICAL_START)
>
> --
> Jens Axboe
>
> -
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--
Coywolf Qi Hunt