Re: [PATCH] mm,x86: limit 32 bit kernel to 12GB memory

[Pierre-Loup A. Griffais]

Reviewed-by: Pierre-Loup A. Griffais <pgriffais@xxxxxxxxxxxxxxxxx>

On 05/01/2013 07:46 PM, Rik van Riel wrote:
> On Wed, 1 May 2013 21:34:26 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > On Mon, Apr 29, 2013 at 08:48:17PM -0400, Rik van Riel wrote:
> > > It could also print out a friendly message, to
> > > inform the user they should upgrade to a 64 bit
> > > kernel to enjoy the use of all of their memory.
> >
> > Oh, oh, oh!!! Can we use my message:
> >
> >    http://lwn.net/Articles/501769/
> >
> > OK, maybe it's not so friendly ;-)
>
> Here's a somewhat friendlier one. Printing out the total amount of
> memory in the system may give them some extra motivation to upgrade
> to a 64 bit kernel :)
>
> ---8<----
> Subject: mm,x86: limit 32 bit kernel to 12GB memory
>
> Running 32 bit kernels on very large memory systems is a recipe
> for disaster, due to fundamental architectural limits in both
> Linux and the hardware. Moreover, all modern hardware with large
> memory supports 64 bits.
>
> However, many users continue using 32 bit kernels, and end up
> encountering stability and performance problems as a result.
>
> It may be better to save those people the frustration of stability
> issues by limiting memory on a 32 bit kernel to 12GB (about the upper
> limit that still works right), and printing a friendly reminder that
> they really should be using a 64 bit kernel.
>
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>   arch/x86/include/asm/setup.h |  1 +
>   arch/x86/mm/init_32.c        | 11 +++++++++++
>   2 files changed, 12 insertions(+)
>
> diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
> index b7bf350..79de6bf 100644
> --- a/arch/x86/include/asm/setup.h
> +++ b/arch/x86/include/asm/setup.h
> @@ -14,6 +14,7 @@
>    */
>   #define MAXMEM_PFN	PFN_DOWN(MAXMEM)
>   #define MAX_NONPAE_PFN	(1 << 20)
> +#define MAX_PAE_PFN	(3 << 20)
>
>   #endif /* __i386__ */
>
> diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> index 3ac7e31..e35b3f5 100644
> --- a/arch/x86/mm/init_32.c
> +++ b/arch/x86/mm/init_32.c
> @@ -600,6 +600,12 @@ static void __init lowmem_pfn_init(void)
>
>   #define MSG_HIGHMEM_TRIMMED \
>   	"Warning: only 4GB will be used. Use a HIGHMEM64G enabled kernel!\n"
> +
> +#define MSG_HIGHMEM_INSANE \
> +	"Warning: 32 bit kernels on large memory systems have problems.\n" \
> +	"Limiting memory to 12GB for system stability.\n" \
> +	"Use a 64 bit kernel to access all %lu MB of memory.\n"
> +
>   /*
>    * We have more RAM than fits into lowmem - we try to put it into
>    * highmem, also taking the highmem=x boot parameter into account:
> @@ -634,6 +640,11 @@ static void __init highmem_pfn_init(void)
>   		max_pfn = MAX_NONPAE_PFN;
>   		printk(KERN_WARNING MSG_HIGHMEM_TRIMMED);
>   	}
> +#else /* !CONFIG_HIGHMEM64G */
> +	if (max_pfn > MAX_PAE_PFN) {
> +		printk(KERN_WARNING MSG_HIGHMEM_INSANE, max_pfn>>8);
> +		max_pfn = MAX_PFN;
> +	}
>   #endif /* !CONFIG_HIGHMEM64G */
>   #endif /* !CONFIG_HIGHMEM */
>   }

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