s390/uaccess: use sane length for __strncpy_from_user()

The average string that is copied from user space to kernel space is
rather short. E.g. booting a system involves about 50.000
strncpy_from_user() calls where the NULL terminated string has an
average size of 27 bytes.

By default our s390 specific strncpy_from_user() implementation
however copies up to 4096 bytes, which is a waste of cpu cycles and
cache lines. Therefore reduce the default length to L1_CACHE_BYTES
(256 bytes), which also reduces the average execution time of
strncpy_from_user() by 30-40%.

Alternatively we could have switched to the generic
strncpy_from_user() implementation, however it turned out that that
variant would be slower than the now optimized s390 variant.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/lib/uaccess.c b/arch/s390/lib/uaccess.c
index 1e5bb2b86c423fefee7987ec45483077748e8ca8..b3bd3f23b8e851c7f67e46cb527e20603512e7f5 100644 (file)
--- a/arch/s390/lib/uaccess.c
+++ b/arch/s390/lib/uaccess.c
@@ -337,8 +337,8 @@ long __strncpy_from_user(char *dst, const char __user *src, long size)
                return 0;
        done = 0;
        do {
-               offset = (size_t)src & ~PAGE_MASK;
-               len = min(size - done, PAGE_SIZE - offset);
+               offset = (size_t)src & (L1_CACHE_BYTES - 1);
+               len = min(size - done, L1_CACHE_BYTES - offset);
                if (copy_from_user(dst, src, len))
                        return -EFAULT;
                len_str = strnlen(dst, len);