mm: disable fault around on emulated access bit architecture

fault_around aims to reduce minor faults of file-backed pages via
speculative ahead pte mapping and relying on readahead logic.  However,
on non-HW access bit architecture the benefit is highly limited because
they should emulate the young bit with minor faults for reclaim's page
aging algorithm.  IOW, we cannot reduce minor faults on those
architectures.

I did quick a test on my ARM machine.

512M file mmap sequential every word read on eSATA drive 4 times.
stddev is stable.

  = fault_around 4096 =
  elapsed time(usec): 6747645

  = fault_around 65536 =
  elapsed time(usec): 6709263

  0.5% gain.

Even when I tested it with eMMC there is no gain because I guess with
slow storage the major fault is the dominant factor.

Also, fault_around has the side effect of shrinking slab more
aggressively and causes higher vmpressure, so if such speculation fails,
it can evict slab more which can result in page I/O (e.g., inode cache).
In the end, it would make void any benefit of fault_around.

So let's make the default "disabled" on those architectures.

Link: http://lkml.kernel.org/r/20160518014229.GB21538@bbox
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2899,8 +2899,16 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }
 
+/*
+ * If architecture emulates "accessed" or "young" bit without HW support,
+ * there is no much gain with fault_around.
+ */
 static unsigned long fault_around_bytes __read_mostly =
+#ifndef __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
+	PAGE_SIZE;
+#else
 	rounddown_pow_of_two(65536);
+#endif
 
 #ifdef CONFIG_DEBUG_FS
 static int fault_around_bytes_get(void *data, u64 *val)