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 <fdmanana@xxxxxxxxx>
---
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
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