Commit 675dfe1223 ("btrfs: fix block group item corruption after
inserting new block group") fixed one race that resulted in not persisting
a block group's item when its "used" bytes field decreases to zero.
However there's another race that can happen in a much shorter time window
that results in the same problem. The following sequence of steps explains
how it can happen:
1) Task A creates a metadata block group X, its "used" and "commit_used"
fields are initialized to 0;
2) Two extents are allocated from block group X, so its "used" field is
updated to 32K, and its "commit_used" field remains as 0;
3) Transaction commit starts, by some task B, and it enters
btrfs_start_dirty_block_groups(). There it tries to update the block
group item for block group X, which currently has its "used" field with
a value of 32K and its "commit_used" field with a value of 0. However
that fails since the block group item was not yet inserted, so at
update_block_group_item(), the btrfs_search_slot() call returns 1, and
then we set 'ret' to -ENOENT. Before jumping to the label 'fail'...
4) The block group item is inserted by task A, when for example
btrfs_create_pending_block_groups() is called when releasing its
transaction handle. This results in insert_block_group_item() inserting
the block group item in the extent tree (or block group tree), with a
"used" field having a value of 32K and setting "commit_used", in struct
btrfs_block_group, to the same value (32K);
5) Task B jumps to the 'fail' label and then resets the "commit_used"
field to 0. At btrfs_start_dirty_block_groups(), because -ENOENT was
returned from update_block_group_item(), we add the block group again
to the list of dirty block groups, so that we will try again in the
critical section of the transaction commit when calling
btrfs_write_dirty_block_groups();
6) Later the two extents from block group X are freed, so its "used" field
becomes 0;
7) If no more extents are allocated from block group X before we get into
btrfs_write_dirty_block_groups(), then when we call
update_block_group_item() again for block group X, we will not update
the block group item to reflect that it has 0 bytes used, because the
"used" and "commit_used" fields in struct btrfs_block_group have the
same value, a value of 0.
As a result after committing the transaction we have an empty block
group with its block group item having a 32K value for its "used" field.
This will trigger errors from fsck ("btrfs check" command) and after
mounting again the fs, the cleaner kthread will not automatically delete
the empty block group, since its "used" field is not 0. Possibly there
are other issues due to this inconsistency.
When this issue happens, the error reported by fsck is like this:
[1/7] checking root items
[2/7] checking extents
block group [1104150528 1073741824] used 39796736 but extent items used 0
ERROR: errors found in extent allocation tree or chunk allocation
(...)
So fix this by not resetting the "commit_used" field of a block group when
we don't find the block group item at update_block_group_item().
Fixes: 7248e0cebb ("btrfs: skip update of block group item if used bytes are the same")
CC: stable@vger.kernel.org # 6.2+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
2d6cd791e6