tree 35131555f323b709ffa0926e0d673f156b3c4428
parent cc2c39d605d23e7f1fbc376bf011fd06e46ee79f
author Johannes Thumshirn <jthumshirn@suse.de> 1543395296 +0100
committer David Sterba <dsterba@suse.com> 1545054701 +0100

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>