btrfs: document extent mapping assumptions in checksum

Document why map_private_extent_buffer() cannot return '1' (i.e. the map
spans two pages) for the csum_tree_block() case.

The current algorithm for detecting a page boundary crossing in
map_private_extent_buffer() will return a '1' *IFF* the extent buffer's
offset in the page + the offset passed in by csum_tree_block() and the
minimal length passed in by csum_tree_block() - 1 are bigger than
PAGE_SIZE.

We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32
and the current extent buffer allocator always guarantees page aligned
extends, so the above condition can't be true.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
 
 	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 (err)