Re: [PATCH] ext4: fix reserved space counter leakage

On 8/22/21 9:06 PM, Joseph Qi wrote:

On 8/21/21 12:45 AM, Eric Whitney wrote:

quoted

* Jeffle Xu [off-list ref]:

quoted

When ext4_es_insert_delayed_block() returns error, e.g., ENOMEM,
previously reserved space is not released as the error handling,
in which case @s_dirtyclusters_counter is left over. Since this delayed
extent failes to be inserted into extent status tree, when inode is
written back, the extra @s_dirtyclusters_counter won't be subtracted and
remains there forever.

This can leads to /sys/fs/ext4/<dev>/delayed_allocation_blocks remains
non-zero even when syncfs is executed on the filesystem.

Hi:

I think the fix below looks fine.  However, this comment doesn't look right
to me.  Are you really seeing delayed_allocation_blocks values that remain
incorrectly elevated across last closes (or across file system unmounts and
remounts)?  s_dirtyclusters_counter isn't written out to stable storage -
it's an in-memory only variable that's created when a file is first opened
and destroyed on last close.

Actually we've encountered a real case in our production environment,
which has about 20G space lost (df - du = ~20G).
After some investigation, we've confirmed that it cause by leaked
s_dirtyclusters_counter (~5M), and even we do manually sync, it remains.
Since there is no error messages, we've checked all logic around
s_dirtyclusters_counter and found this. Also we can manually inject
error and reproduce the leaked s_dirtyclusters_counter.

BTW, it's a runtime lost, but not about on-disk.
If umount and then mount it again, it becomes normal. But
application also should be restarted...

Thanks,
Joseph