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>

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 4bc270ef29b4ed8da59cbf4f4af30a618ef0c7f2..14d355d0cb7a439177884f0d88b50831b6a7a6a9 100644 (file)
--- a/fs/btrfs/disk-io.c
+++ b/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)