On crash memory test rewrote so that it actaully works.

1) We no longer test location by location, otherwise the CPU write cache
completely makes our business useless.
2) We still need a memory test that operates in steps from the first to
the last location in order to never hit the cache, but that is still
able to retain the memory content.

This was tested using a Linux box containing a bad memory module with a
zingle bit error (always zero).

So the final solution does has an error propagation step that is:

1) Invert bits at every location.
2) Swap adiacent locations.
3) Swap adiacent locations again.
4) Invert bits at every location.
5) Swap adiacent locations.
6) Swap adiacent locations again.

Before and after these steps, and after step 4, a CRC64 checksum is computed.
If the three CRC64 checksums don't match, a memory error was detected.

src/debug.c

@@ -667,16 +667,22 @@ void logCurrentClient(void) {
 }
 
 #if defined(HAVE_PROC_MAPS)
-int memtest_non_destructive(void *addr, size_t size); /* memtest.c */
+uint64_t crc64(uint64_t crc, const unsigned char *s, uint64_t l);
+void memtest_non_destructive_invert(void *addr, size_t size);
+void memtest_non_destructive_swap(void *addr, size_t size);
+#define MEMTEST_MAX_REGIONS 128
 
 int memtest_test_linux_anonymous_maps(void) {
     FILE *fp = fopen("/proc/self/maps","r");
     char line[1024];
     size_t start_addr, end_addr, size;
+    size_t start_vect[MEMTEST_MAX_REGIONS];
+    size_t size_vect[MEMTEST_MAX_REGIONS];
+    int regions = 0, j;
+    uint64_t crc1 = 0, crc2 = 0, crc3 = 0;
 
     while(fgets(line,sizeof(line),fp) != NULL) {
         char *start, *end, *p = line;
-        int j;
 
         start = p;
         p = strchr(p,'-');
@@ -695,17 +701,51 @@ int memtest_test_linux_anonymous_maps(void) {
         start_addr = strtoul(start,NULL,16);
         end_addr = strtoul(end,NULL,16);
         size = end_addr-start_addr;
-        redisLog(REDIS_WARNING,
-            "Testing memory at %lx (%lu bytes)", start_addr, size);
-        for (j = 0; j < 3; j++) {
-            if (memtest_non_destructive((void*)start_addr,size) != 0) {
-                fclose(fp);
-                return 1;
-            }
-        }
+
+        start_vect[regions] = start_addr;
+        size_vect[regions] = size;
+        printf("Testing %lx %lu\n", start_vect[regions], size_vect[regions]);
+        regions++;
     }
+
+    /* Test all the regions as an unique sequential region.
+     * 1) Take the CRC64 of the memory region. */
+    for (j = 0; j < regions; j++) {
+        crc1 = crc64(crc1,(void*)start_vect[j],size_vect[j]);
+    }
+
+    /* 2) Invert bits, swap adiacent words, swap again, invert bits.
+     * This is the error amplification step. */
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_invert((void*)start_vect[j],size_vect[j]);
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_swap((void*)start_vect[j],size_vect[j]);
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_swap((void*)start_vect[j],size_vect[j]);
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_invert((void*)start_vect[j],size_vect[j]);
+
+    /* 3) Take the CRC64 sum again. */
+    for (j = 0; j < regions; j++)
+        crc2 = crc64(crc2,(void*)start_vect[j],size_vect[j]);
+
+    /* 4) Swap + Swap again */
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_swap((void*)start_vect[j],size_vect[j]);
+    for (j = 0; j < regions; j++)
+        memtest_non_destructive_swap((void*)start_vect[j],size_vect[j]);
+
+    /* 5) Take the CRC64 sum again. */
+    for (j = 0; j < regions; j++)
+        crc3 = crc64(crc3,(void*)start_vect[j],size_vect[j]);
+
+    /* NOTE: It is very important to close the file descriptor only now
+     * because closing it before may result into unmapping of some memory
+     * region that we are testing. */
     fclose(fp);
-    return 0;
+
+    /* If the two CRC are not the same, we trapped a memory error. */
+    return crc1 != crc2 || crc2 != crc3;
 }
 #endif