btrfs: simplify tree block checksumming loop

Thw whole point of csum_tree_block is to iterate over all extent buffer
pages and pass it to checksumming functions. The bytes where checksum is
stored must be skipped, thus map_private_extent_buffer. This complicates
further offset calculations.

As the first page will be always present, checksum the relevant bytes
unconditionally and then do a simple iteration over the remaining pages.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -259,38 +259,22 @@ struct extent_map *btree_get_extent(struct btrfs_inode *inode,
 static int csum_tree_block(struct extent_buffer *buf, u8 *result)
 {
 	struct btrfs_fs_info *fs_info = buf->fs_info;
+	const int num_pages = fs_info->nodesize >> PAGE_SHIFT;
 	SHASH_DESC_ON_STACK(shash, fs_info->csum_shash);
-	unsigned long len;
-	unsigned long cur_len;
-	unsigned long offset = BTRFS_CSUM_SIZE;
 	char *kaddr;
-	unsigned long map_start;
-	unsigned long map_len;
-	int err;
+	int i;
 
 	shash->tfm = fs_info->csum_shash;
 	crypto_shash_init(shash);
+	kaddr = page_address(buf->pages[0]);
+	crypto_shash_update(shash, kaddr + BTRFS_CSUM_SIZE,
+			    PAGE_SIZE - BTRFS_CSUM_SIZE);
 
-	len = buf->len - offset;
-
-	while (len > 0) {
-		/*
-		 * Note: we don't need to check for the err == 1 case here, as
-		 * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
-		 * and 'min_len = 32' and the currently implemented mapping
-		 * algorithm we cannot cross a page boundary.
-		 */
-		err = map_private_extent_buffer(buf, offset, 32,
-					&kaddr, &map_start, &map_len);
-		if (WARN_ON(err))
-			return err;
-		cur_len = min(len, map_len - (offset - map_start));
-		crypto_shash_update(shash, kaddr + offset - map_start, cur_len);
-		len -= cur_len;
-		offset += cur_len;
+	for (i = 1; i < num_pages; i++) {
+		kaddr = page_address(buf->pages[i]);
+		crypto_shash_update(shash, kaddr, PAGE_SIZE);
 	}
 	memset(result, 0, BTRFS_CSUM_SIZE);
-
 	crypto_shash_final(shash, result);
 
 	return 0;