s390/mm: align 64-bit PIE binaries to 4GB

The base address (STACK_TOP / 3 * 2) for a 64-bit program is two thirds into the 4GB segment at 0x2aa00000000. The randomization added on z13 can eat another 1GB of the remaining 1.33GB to the next 4GB boundary. In the worst case 300MB are left for the executable + bss which may cross into the next 4GB segment. This is bad for branch prediction, therefore align the base address to 4GB to give the program more room before it crosses the 4GB boundary. Signed-off-by: Martin Schwidefsky <[email protected]>
wentasah · Feb 19, 2015 · 4ba2815 · 4ba2815
1 parent f4dce5c
commit 4ba2815
Showing 1 changed file with 4 additions and 1 deletion.
diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
@@ -183,7 +183,10 @@ unsigned long randomize_et_dyn(void)
 {
 	unsigned long base;
 
-	base = (STACK_TOP / 3 * 2) & (~mmap_align_mask << PAGE_SHIFT);
+	base = STACK_TOP / 3 * 2;
+	if (!is_32bit_task())
+		/* Align to 4GB */
+		base &= ~((1UL << 32) - 1);
 	return base + mmap_rnd();
 }