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

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 8/21/21 12:45 AM, Eric Whitney wrote:
> * Jeffle Xu <jefflexu@linux.alibaba.com>:
>> 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.

Thanks,
Joseph