[PATCH 1/6 -v2] libext2fs: optimize rb_set_bmap_range()

[Theodore Ts'o <tytso@mit.edu>]

This speeds up reading bitmaps from disk for very large (and full)
disks by significant amounts (i.e., up to two CPU minutes for a 4T
file system).

Addresses-Google-Bug: #7534813

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 lib/ext2fs/blkmap64_rb.c | 32 +++++++++++++++++++++++++++++---
 1 file changed, 29 insertions(+), 3 deletions(-)

diff --git a/lib/ext2fs/blkmap64_rb.c b/lib/ext2fs/blkmap64_rb.c
index a42eda1..0fc7c57 100644
--- a/lib/ext2fs/blkmap64_rb.c
+++ b/lib/ext2fs/blkmap64_rb.c
@@ -674,16 +674,42 @@ static errcode_t rb_set_bmap_range(ext2fs_generic_bitmap bitmap,
 				     __u64 start, size_t num, void *in)
 {
 	struct ext2fs_rb_private *bp;
+	unsigned char *cp = in;
 	size_t i;
+	int first_set = -1;
 	int ret;
 
 	bp = (struct ext2fs_rb_private *) bitmap->private;
 
 	for (i = 0; i < num; i++) {
-		ret = ext2fs_test_bit(i, in);
-		if (ret)
-			rb_insert_extent(start + i - bitmap->start, 1, bp);
+		if (i & 7 == 0) {
+			unsigned char c = cp[i/8];
+			if (c == 0xFF) {
+				if (first_set == -1)
+					first_set = i;
+				i += 7;
+				continue;
+			}
+			if ((c == 0x00) && (first_set == -1)) {
+				i += 7;
+				continue;
+			}
+		}
+		if (ext2fs_test_bit(i, in)) {
+			if (first_set == -1)
+				first_set = i;
+			continue;
+		}
+		if (first_set == -1)
+			continue;
+
+		rb_insert_extent(start + first_set - bitmap->start,
+				 i - first_set, bp);
+		first_set = -1;
 	}
+	if (first_set != -1)
+		rb_insert_extent(start + first_set - bitmap->start,
+				 num - first_set, bp);
 
 	return 0;
 }
-- 
1.7.12.rc0.22.gcdd159b