[POWERPC] update_mmu_cache: Don't cache-flush non-readable pages

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.

This fixes a bug reported by some users where the kernel would crash
while dumping core on a threaded program.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

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);