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 <schwidefsky@de.ibm.com>

diff --git a/arch/s390/mm/mmap.c b/arch/s390/mm/mmap.c
index d008f638b2cd27f7615c11a02bb5d2a8f3138d04..179a2c20b01f143a51a897d9b85e33198bf881a7 100644 (file)
--- 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();
 }