On 04/24/2018 02:17 PM, Qu Wenruo wrote:
On 2018年04月24日 13:52, Su Yue wrote:
For an extent item which contains many tree block backrefs, like
=================================================================
In 020-extent-ref-cases/keyed_block_ref.img
item 10 key (29470720 METADATA_ITEM 0) itemoff 3450 itemsize 222
refs 23 gen 10 flags TREE_BLOCK
tree block skinny level 0
tree block backref root 278
tree block backref root 277
tree block backref root 276
tree block backref root 275
tree block backref root 274
tree block backref root 273
tree block backref root 272
tree block backref root 271
tree block backref root 270
tree block backref root 269
tree block backref root 268
tree block backref root 267
tree block backref root 266
tree block backref root 265
tree block backref root 264
tree block backref root 263
tree block backref root 262
tree block backref root 261
tree block backref root 260
tree block backref root 259
tree block backref root 258
tree block backref root 257
=================================================================
In find_parent_nodes(), these refs's parents are 0, then __merge_refs
will merge refs to one ref. It causes only one root to be returned.
This is a pretty big problem, and it would cause qgroup verification
code to cause false alerts.
So a new test case would do great help here.
Let me think how to construct a test case a while.
If I remember correctly, the function is called rarely only
in check/lowmem check.
Check calls it in a very corner case.
As you know, extent check in lowmme is buggy which I'm try to fix.
So, if both parents are 0, do not merge refs.
Lowmem check calls find_parent_nodes frequently to decide whether.
check an extent buffer or not. These bug influences bytes accounting.
Although it looks good so far, and would fix the problem you found, but
Fixing another bug in lowmem, found it by accident.
I strongly recommend to port kernel backref code to btrfs-progs here, as
kernel backref code is more tested than btrfs-progs backref code.
It's a fact. Seen backref.c in btrfs-progs, there are many useless codes
and confusing logicals.
I agree that port kernel backref code to btrfs-progs.
I'm pretty sure we have more bugs in btrfs-progs backref, and I'm not a
big fan of playing whac-a-mole here.
Signed-off-by: Su Yue <suy.f...@cn.fujitsu.com>
---
backref.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/backref.c b/backref.c
index 51553c702187..daadb6299c79 100644
--- a/backref.c
+++ b/backref.c
@@ -507,6 +507,8 @@ static void __merge_refs(struct pref_state *prefstate, int
mode)
} else {
if (ref1->parent != ref2->parent)
continue;
+ if (ref1->parent == 0)
+ continue;
It's better to put it above (ref1->parent != ref2->parent).
As parent == 0 means we haven't resolve the parent bytenr yet, so can't
be merged nor compared.
Ok, will do it in V2.
Thanks,
Su
Thanks,
Qu
}
eie = ref1->inode_list;
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