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 product of 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.
Signed-off-by: Johannes Thumshirn <jthumsh...@suse.de>
---
fs/btrfs/disk-io.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 4bc270ef29b4..14d355d0cb7a 100644
--- 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)
--
2.16.4
