[PATCH] update_mmu_cache: Don't dcbst non-readable pages.

[Benjamin Herrenschmidt]

On Tue, 2008-02-05 at 13:43 -0600, Scott Wood wrote:
> Currently, update_mmu_cache will crash if given a no-access PTE.  There's no
> need to synchronize dcache/icache unless it's an exec mapping -- however,
> due to the existence of older glibc versions that execute out of a
> read-but-no-exec page, readability is tested instead.
> 
> This assumes no exec-only mappings; if such mappings become supported, they
> will need to go through the kmap_atomic() version of dcache/icache
> synchronization.
> 
> Signed-off-by: Scott Wood <scottwood at freescale.com>

Acked-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> ---
> This fixes the following bug:
> http://ozlabs.org/pipermail/linuxppc-dev/2008-February/051080.html
> 
>  arch/powerpc/mm/mem.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index e812244..c7d7bd4 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -483,7 +483,12 @@ void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
>  		 */
>  		_tlbie(address, 0 /* 8xx doesn't care about PID */);
>  #endif
> -		if (!PageReserved(page)
> +		/* The _PAGE_USER test should really be _PAGE_EXEC, but
> +		 * older glibc versions execute some code from no-exec
> +		 * pages, which for now we are supporting.  If exec-only
> +		 * pages are ever implemented, this will have to change.
> +		 */
> +		if (!PageReserved(page) && (pte_val(pte) & _PAGE_USER)
>  		    && !test_bit(PG_arch_1, &page->flags)) {
>  			if (vma->vm_mm == current->active_mm) {
>  				__flush_dcache_icache((void *) address);