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Btrfs: correctly determine if blocks are shared in btrfs_compare_trees

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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@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fb.com>
This commit is contained in:
Filipe Manana 2014-02-20 21:15:25 +00:00 committed by Josef Bacik
parent 9dc442143b
commit 6baa4293af
1 changed files with 10 additions and 1 deletions
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11
fs/btrfs/ctree.c
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@ -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.