[PATCH] Remove bitmap_shift_*() bitmap length limits

From: William Lee Irwin III <wli@holomorphy.com>

Chang bitmap_shift_left()/bitmap_shift_right() to have O(1) stackspace
requirements.

Given zeroed tail preconditions these implementations satisfy zeroed tail
postconditions, which makes them compatible with whatever changes from Paul
Jackson one may want to merge in the future.  No particular effort was
required to ensure this.

A small (but hopefully forgiveable) cleanup is a spelling correction:
s/bitmap_shift_write/bitmap_shift_right/ in one of the kerneldoc comments.

The primary effect of the patch is to remove the MAX_BITMAP_BITS
limitation, so restoring the NR_CPUS to be limited only by stackspace and
slab allocator maximums.  They also look vaguely more efficient than the
current code, though as this was not done for performance reasons, no
performance testing was done.

lib/bitmap.c

@@ -12,8 +12,6 @@
 #include <asm/bitops.h>
 #include <asm/uaccess.h>
 
-#define MAX_BITMAP_BITS	512U	/* for ia64 NR_CPUS maximum */
-
 int bitmap_empty(const unsigned long *bitmap, int bits)
 {
 	int k, lim = bits/BITS_PER_LONG;
@@ -72,7 +70,7 @@ void bitmap_complement(unsigned long *bitmap, int bits)
 EXPORT_SYMBOL(bitmap_complement);
 
 /*
- * bitmap_shift_write - logical right shift of the bits in a bitmap
+ * bitmap_shift_right - logical right shift of the bits in a bitmap
  *   @dst - destination bitmap
  *   @src - source bitmap
  *   @nbits - shift by this many bits
@@ -85,15 +83,32 @@ EXPORT_SYMBOL(bitmap_complement);
 void bitmap_shift_right(unsigned long *dst,
 			const unsigned long *src, int shift, int bits)
 {
-	int k;
-	DECLARE_BITMAP(__shr_tmp, MAX_BITMAP_BITS);
-
-	BUG_ON(bits > MAX_BITMAP_BITS);
-	bitmap_clear(__shr_tmp, bits);
-	for (k = 0; k < bits - shift; ++k)
-		if (test_bit(k + shift, src))
-			set_bit(k, __shr_tmp);
-	bitmap_copy(dst, __shr_tmp, bits);
+	int k, lim = BITS_TO_LONGS(bits), left = bits % BITS_PER_LONG;
+	int off = shift/BITS_PER_LONG, rem = shift % BITS_PER_LONG;
+	unsigned long mask = (1UL << left) - 1;
+	for (k = 0; off + k < lim; ++k) {
+		unsigned long upper, lower;
+
+		/*
+		 * If shift is not word aligned, take lower rem bits of
+		 * word above and make them the top rem bits of result.
+		 */
+		if (!rem || off + k + 1 >= lim)
+			upper = 0;
+		else {
+			upper = src[off + k + 1];
+			if (off + k + 1 == lim - 1 && left)
+				upper &= mask;
+		}
+		lower = src[off + k];
+		if (left && off + k == lim - 1)
+			lower &= mask;
+		dst[k] = upper << (BITS_PER_LONG - rem) | lower >> rem;
+		if (left && k == lim - 1)
+			dst[k] &= mask;
+	}
+	if (off)
+		memset(&dst[lim - off], 0, off*sizeof(unsigned long));
 }
 EXPORT_SYMBOL(bitmap_shift_right);
 
@@ -111,15 +126,28 @@ EXPORT_SYMBOL(bitmap_shift_right);
 void bitmap_shift_left(unsigned long *dst,
 			const unsigned long *src, int shift, int bits)
 {
-	int k;
-	DECLARE_BITMAP(__shl_tmp, MAX_BITMAP_BITS);
-
-	BUG_ON(bits > MAX_BITMAP_BITS);
-	bitmap_clear(__shl_tmp, bits);
-	for (k = bits; k >= shift; --k)
-		if (test_bit(k - shift, src))
-			set_bit(k, __shl_tmp);
-	bitmap_copy(dst, __shl_tmp, bits);
+	int k, lim = BITS_TO_LONGS(bits), left = bits % BITS_PER_LONG;
+	int off = shift/BITS_PER_LONG, rem = shift % BITS_PER_LONG;
+	for (k = lim - off - 1; k >= 0; --k) {
+		unsigned long upper, lower;
+
+		/*
+		 * If shift is not word aligned, take upper rem bits of
+		 * word below and make them the bottom rem bits of result.
+		 */
+		if (rem && k > 0)
+			lower = src[k - 1];
+		else
+			lower = 0;
+		upper = src[k];
+		if (left && k == lim - 1)
+			upper &= (1UL << left) - 1;
+		dst[k + off] = lower  >> (BITS_PER_LONG - rem) | upper << rem;
+		if (left && k + off == lim - 1)
+			dst[k + off] &= (1UL << left) - 1;
+	}
+	if (off)
+		memset(dst, 0, off*sizeof(unsigned long));
 }
 EXPORT_SYMBOL(bitmap_shift_left);