Subject: Re: [PATCH 1/4] [tip:x86/mm] Correcting improper large page preservation

From: Suresh Siddha <[email-masked]>
Date: Fri, 02 Apr 2010 23:43:20 -0700

On Wed, 2010-03-31 at 18:59 -0700, Siarhei Liakh wrote:
> +	/*
> +	 * .data and .bss should always be writable.
> +	 */
> +	if ((within(pfn, __pa((unsigned long)_sdata) >> PAGE_SHIFT,
> +		   __pa((unsigned long)_edata) >> PAGE_SHIFT)) ||
> +	    (within(pfn, __pa((unsigned long)__bss_start) >> PAGE_SHIFT,
> +		   __pa((unsigned long)__bss_stop) >> PAGE_SHIFT))) {
> +		pgprot_val(required) |= _PAGE_RW;
> +	}

I have reviewed this patch and the only comment I have is:

On 64bit kernels, kernel text/data mapping and kernel identity mappings
are different virtual addresses mapping to same pfn ranges. For the
data/bss pages, does it help (in identifying certain data corruptions
more easily) in making the kernel identity mapping to be set to
read-only and enforce the need of RW only for the kernel data mappings.

Or is there some obscure code that uses something like
__va(__pa(data_symbol)) and writes to it?

If not, we can remove the __pa() constructs above and use the addr for
comparisons.

Otherwise this patch looks good to me.

Reviewed-by: Suresh Siddha <[email-masked]>