x86: memtest: reuse test patterns when memtest parameter exceeds number of available patterns

Impact: fix unexpected behaviour when pattern number is out of range

Current implementation provides 4 patterns for memtest. The code doesn't
check whether the memtest parameter value exceeds the maximum pattern number.

Instead the memtest code pretends to test with non-existing patterns, e.g.
when booting with memtest=10 I've observed the following

  ...
  early_memtest: pattern num 10
  0000001000 - 0000006000 pattern 0
  ...
  0000001000 - 0000006000 pattern 1
  ...
  0000001000 - 0000006000 pattern 2
  ...
  0000001000 - 0000006000 pattern 3
  ...
  0000001000 - 0000006000 pattern 4
  ...
  0000001000 - 0000006000 pattern 5
  ...
  0000001000 - 0000006000 pattern 6
  ...
  0000001000 - 0000006000 pattern 7
  ...
  0000001000 - 0000006000 pattern 8
  ...
  0000001000 - 0000006000 pattern 9
  ...

But in fact Linux didn't test anything for patterns > 4 as the default
case in memtest() is to leave the function.

I suggest to use the memtest parameter as the number of tests to be
performed and to re-iterate over all existing patterns.
Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/memtest.c

@@ -9,6 +9,8 @@
 
 #include <asm/e820.h>
 
+#define _MAX_MEM_PATTERNS 4
+
 static void __init memtest(unsigned long start_phys, unsigned long size,
 				 unsigned pattern)
 {
@@ -21,6 +23,8 @@ static void __init memtest(unsigned long start_phys, unsigned long size,
 	unsigned long count;
 	unsigned long incr;
 
+	pattern = pattern % _MAX_MEM_PATTERNS;
+
 	switch (pattern) {
 	case 0:
 		val = 0UL;
@@ -111,7 +115,8 @@ void __init early_memtest(unsigned long start, unsigned long end)
 
 			printk(KERN_CONT "\n  %010llx - %010llx pattern %d",
 			       (unsigned long long)t_start,
-				(unsigned long long)t_start + t_size, pattern);
+			       (unsigned long long)t_start + t_size,
+			       pattern % _MAX_MEM_PATTERNS);
 
 			memtest(t_start, t_size, pattern);