Btrfs lowmem check reports the following false alert:
------
ERROR: file extent[267 2162688] root 256 owner 5 backref lost
------
The file extent is in the leaf which is shared by file tree 256 and fs
tree.
------
leaf 30605312 items 46 free space 4353 generation 7 owner 5
......
item 45 key (267 EXTENT_DATA 2162688) itemoff 5503 itemsize 53
generation 7 type 2 (prealloc)
prealloc data disk byte 13631488 nr 65536
prealloc data offset 32768 nr 32768
------
And there is the corresponding extent_data_ref item in the extent tree.
------
item 1 key (13631488 EXTENT_DATA_REF 1007496934287921081) itemoff 15274 itemsize 28
extent data backref root 5 objectid 267 offset 2129920 count 1
------
The offset of EXTENT_DATA_REF which is the hash of the owner root objectid,
the inode number and the calculated offset (file offset - extent offset).
What caused the false alert is the code mix up the owner root objectid and
the file tree objectid.
Fixes: b0d360b541f0 ("btrfs-progs: check: introduce function to check data backref in extent tree")
Signed-off-by: Lu Fengqi <lufq.fnst@xxxxxxxxxxxxxx>
---
check/mode-lowmem.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/check/mode-lowmem.c b/check/mode-lowmem.c
index f37b1b2c1571..6f1ea8db341d 100644
--- a/check/mode-lowmem.c
+++ b/check/mode-lowmem.c
@@ -2689,8 +2689,8 @@ static int check_extent_data_item(struct btrfs_root *root,
/* Didn't find inlined data backref, try EXTENT_DATA_REF_KEY */
dbref_key.objectid = btrfs_file_extent_disk_bytenr(eb, fi);
dbref_key.type = BTRFS_EXTENT_DATA_REF_KEY;
- dbref_key.offset = hash_extent_data_ref(root->objectid,
- fi_key.objectid, fi_key.offset - offset);
+ dbref_key.offset = hash_extent_data_ref(owner, fi_key.objectid,
+ fi_key.offset - offset);
ret = btrfs_search_slot(NULL, root->fs_info->extent_root,
&dbref_key, &path, 0, 0);
--
2.16.2
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