2012/5/23 Jan Kara [off-list ref]:

On Sun 20-05-12 22:23:14, Akinobu Mita wrote:

quoted

memweight() is the function that counts the total number of bits set
in memory area.  The memory area doesn't need to be aligned to
long-word boundary unlike bitmap_weight().

 Thanks for the patch. I have some comments below.

Thanks for the review.

quoted

@@ -824,3 +825,39 @@ void *memchr_inv(const void *start, int c, size_t bytes)

      return check_bytes8(start, value, bytes % 8);
 }
 EXPORT_SYMBOL(memchr_inv);
+
+/**
+ * memweight - count the total number of bits set in memory area
+ * @ptr: pointer to the start of the area
+ * @bytes: the size of the area
+ */
+size_t memweight(const void *ptr, size_t bytes)
+{
+     size_t w = 0;
+     size_t longs;
+     union {
+             const void *ptr;
+             const unsigned char *b;
+             unsigned long address;
+     } bitmap;

 Ugh, this is ugly and mostly unnecessary. Just use "const unsigned char
*bitmap".

quoted

+
+     for (bitmap.ptr = ptr; bytes > 0 && bitmap.address % sizeof(long);
+                     bytes--, bitmap.address++)
+             w += hweight8(*bitmap.b);

 This can be:
       count = ((unsigned long)bitmap) % sizeof(long);

The count should be the size of unaligned area and it can be greater than
bytes. So

        count = min(bytes,
                    sizeof(long) - ((unsigned long)bitmap) % sizeof(long));

       while (count--) {
               w += hweight(*bitmap);
               bitmap++;
               bytes--;
       }

quoted

+
+     for (longs = bytes / sizeof(long); longs > 0; ) {
+             size_t bits = min_t(size_t, INT_MAX & ~(BITS_PER_LONG - 1),
+                                     longs * BITS_PER_LONG);

 I find it highly unlikely that someone would have such a large bitmap
(256 MB or more on 32-bit). Also the condition as you wrote it can just
overflow so it won't have the desired effect. Just do
       BUG_ON(longs >= ULONG_MAX / BITS_PER_LONG);

The bits argument of bitmap_weight() is int type. So this should be

        BUG_ON(longs >= INT_MAX / BITS_PER_LONG);

and remove the loop completely. If someone comes with such a huge bitmap,
the code can be modified easily (after really closely inspecting whether
such a huge bitmap is really well justified).

size_t memweight(const void *ptr, size_t bytes)
{
	size_t w = 0;
	size_t longs;
	const unsigned char *bitmap = ptr;

	for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
			bytes--, bitmap++)
		w += hweight8(*bitmap);

	longs = bytes / sizeof(long);
	BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
	w += bitmap_weight((unsigned long *)bitmap, longs * BITS_PER_LONG);
	bytes -= longs * sizeof(long);
	bitmap += longs * sizeof(long);

	for (; bytes > 0; bytes--, bitmap++)
		w += hweight8(*bitmap);

	return w;
}