Re: [PATCH] ext4: silence the warning when evicting inode with dioread_nolock

[Zhang Yi <yi.zhang@huawei.com>]

Hi, Jan.

On 2022/6/24 20:51, Jan Kara wrote:
> On Fri 24-06-22 15:04:04, Zhang Yi wrote:
>> When evicting an inode with default dioread_nolock, it could be raced by
>> the unwritten extents converting kworker after writeback some new
>> allocated dirty blocks. It convert unwritten extents to written, the
>> extents could be merged to upper level and free extent blocks, so it
>> could mark the inode dirty again even this inode has been marked
>> I_FREEING. But the inode->i_io_list check and warning in
>> ext4_evict_inode() missing this corner case. Fortunately,
>> ext4_evict_inode() will wait all extents converting finished before this
>> check, so it will not lead to inode use-after-free problem, so every
>> thing is OK besides this warning, let the WARN_ON_ONCE know the
>> dioread_nolock case to silence this warning is fine.
>>
>>  ======
>>  WARNING: CPU: 7 PID: 1092 at fs/ext4/inode.c:227
>>  ext4_evict_inode+0x875/0xc60
>>  ...
>>  RIP: 0010:ext4_evict_inode+0x875/0xc60
>>  ...
>>  Call Trace:
>>   <TASK>
>>   evict+0x11c/0x2b0
>>   iput+0x236/0x3a0
>>   do_unlinkat+0x1b4/0x490
>>   __x64_sys_unlinkat+0x4c/0xb0
>>   do_syscall_64+0x3b/0x90
>>   entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>  RIP: 0033:0x7fa933c1115b
>>  ======
>>
>> rm                          kworker
>>                             ext4_end_io_end()
>> vfs_unlink()
>>  ext4_unlink()
>>                              ext4_convert_unwritten_io_end_vec()
>>                               ext4_convert_unwritten_extents()
>>                                ext4_map_blocks()
>>                                 ext4_ext_map_blocks()
>>                                  ext4_ext_try_to_merge_up()
>>                                   __mark_inode_dirty()
>>                                    check !I_FREEING
>>                                    locked_inode_to_wb_and_lock_list()
>>  iput()
>>   iput_final()
>>    evict()
>>     ext4_evict_inode()
>>      truncate_inode_pages_final() //wait release io_end
>>                                     inode_io_list_move_locked()
>>                              ext4_release_io_end()
>>      trigger WARN_ON_ONCE()
>>
>> Fixes: ceff86fddae8 ("ext4: Avoid freeing inodes on dirty list")
>> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> 
> Good catch! So for the i_nlink == 0 case below, I'd just remove the
> WARN_ON_ONCE altogether. It isn't very useful after your change anyway. But

Yes, indeed. dioread_nolock is enabled by default now, other cases would
seldom happen, and the usage of this WARN_ON_ONCE seems just find the mistaken
case of marking I_FREEING inode dirty without hold inode refcount. But it seems
we haven't found one, no? So just remove this WARN_ON_ONCE is fine in the
i_nlink == 0 case.

> probably we should add:
> 
> 	WARN_ON_ONCE(!list_empty(&inode->i_io_list));
> 
> to the no_delete: case of ext4_evict_inode()? Race like you mention above
> does not seem possible for that case but seeing the complicated
> interactions I'd rather have the assertion in place.
> 

For the no_delete case, I did some tests and IIUC, it's true that the race could
not happen, because inode_lru_isolate() make sure inode->i_data.nrpages is zero
before adding inode into the freeable list, so the evict() procedure could not be
invoked before the page cache have been dropped (it could only happened after
ext4_end_io_end() has been finished).

We don't have a !list_empty(&inode->i_io_list) check for the no_delete case now.
But I am not quite get the purpose of adding it, do you want to detect inode
use-after-free issue in advance?

Thanks,
Yi.

>> ---
>>  fs/ext4/inode.c | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index 3dce7d058985..3b64d72416b7 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -220,11 +220,14 @@ void ext4_evict_inode(struct inode *inode)
>>  
>>  	/*
>>  	 * For inodes with journalled data, transaction commit could have
>> -	 * dirtied the inode. Flush worker is ignoring it because of I_FREEING
>> -	 * flag but we still need to remove the inode from the writeback lists.
>> +	 * dirtied the inode. And for inodes with dioread_nolock, unwritten
>> +	 * extents converting worker could merged extents and also have dirtied
>> +	 * the inode. Flush worker is ignoring it because of I_FREEING flag but
>> +	 * we still need to remove the inode from the writeback lists.
>>  	 */
>>  	if (!list_empty_careful(&inode->i_io_list)) {
>> -		WARN_ON_ONCE(!ext4_should_journal_data(inode));
>> +		WARN_ON_ONCE(!ext4_should_journal_data(inode) &&
>> +			     !ext4_should_dioread_nolock(inode));
>>  		inode_io_list_del(inode);
>>  	}
>>  
>> -- 
>> 2.31.1
>>