Just comparing the pointers (logical disk addresses) of the btree nodes is not completely bullet proof, we have to check if their generation numbers match too.
It is guaranteed that a COW operation will result in a block with a different logical disk address than the original block's address, but over time we can reuse that former logical disk address. For example, creating a 2Gb filesystem on a loop device, and having a script running in a loop always updating the access timestamp of a file, resulted in the same logical disk address being reused for the same fs btree block in about only 4 minutes. This could make us skip entire subtrees when doing an incremental send (which is currently the only user of btrfs_compare_trees). However the odds of getting 2 blocks at the same tree level, with the same logical disk address, equal first slot keys and different generations, should hopefully be very low. Signed-off-by: Filipe David Borba Manana <[email protected]> --- fs/btrfs/ctree.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index cbd3a7d..88d1b1e 100644 --- a/fs/btrfs/ctree.c +++ b/fs/btrfs/ctree.c @@ -5376,6 +5376,8 @@ int btrfs_compare_trees(struct btrfs_root *left_root, int advance_right; u64 left_blockptr; u64 right_blockptr; + u64 left_gen; + u64 right_gen; u64 left_start_ctransid; u64 right_start_ctransid; u64 ctransid; @@ -5640,7 +5642,14 @@ int btrfs_compare_trees(struct btrfs_root *left_root, right_blockptr = btrfs_node_blockptr( right_path->nodes[right_level], right_path->slots[right_level]); - if (left_blockptr == right_blockptr) { + left_gen = btrfs_node_ptr_generation( + left_path->nodes[left_level], + left_path->slots[left_level]); + right_gen = btrfs_node_ptr_generation( + right_path->nodes[right_level], + right_path->slots[right_level]); + if (left_blockptr == right_blockptr && + left_gen == right_gen) { /* * As we're on a shared block, don't * allow to go deeper. -- 1.7.9.5 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
