From d2e174d5d3eedb3b9dd97167307bf1f01bd2fa5c Mon Sep 17 00:00:00 2001 From: Johannes Thumshirn Date: Wed, 28 Nov 2018 09:54:56 +0100 Subject: [PATCH] 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 Signed-off-by: Johannes Thumshirn Signed-off-by: David Sterba --- 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)