Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino

[Chao Yu <yuchao0@huawei.com>]

On 2020/5/7 6:36, Gao Xiang wrote:
> On Wed, May 06, 2020 at 12:16:13PM -0700, Eric Biggers wrote:
>> On Wed, May 06, 2020 at 02:47:19PM +0800, Gao Xiang wrote:
>>> On Wed, May 06, 2020 at 09:58:22AM +0800, Gao Xiang wrote:
>>>> On Tue, May 05, 2020 at 06:24:28PM -0700, Eric Biggers wrote:
>>>>> On Wed, May 06, 2020 at 08:14:07AM +0800, Gao Xiang wrote:
>>>>>>>
>>>>>>> Actually, I think this is wrong because the fsync can be done via a file
>>>>>>> descriptor that was opened to a now-deleted link to the file.
>>>>>>
>>>>>> I'm still confused about this...
>>>>>>
>>>>>> I don't know what's wrong with this version from my limited knowledge?
>>>>>>  inode itself is locked when fsyncing, so
>>>>>>
>>>>>>    if the fsync inode->i_nlink == 1, this inode has only one hard link
>>>>>>    (not deleted yet) and should belong to a single directory; and
>>>>>>
>>>>>>    the only one parent directory would not go away (not deleted as well)
>>>>>>    since there are some dirents in it (not empty).
>>>>>>
>>>>>> Could kindly explain more so I would learn more about this scenario?
>>>>>> Thanks a lot!
>>>>>
>>>>> i_nlink == 1 just means that there is one non-deleted link.  There can be links
>>>>> that have since been deleted, and file descriptors can still be open to them.
>>>>
>>>> Thanks for your inspiration. You are right, thanks.
>>>>
>>>> Correct my words... I didn't check f2fs code just now, it seems f2fs doesn't
>>>> take inode_lock as some other fs like __generic_file_fsync or ubifs_fsync.
>>>>
>>>> And i_sem locks nlink / try_to_fix_pino similarly in some extent. It seems
>>>> no race by using d_find_alias here. Thanks again.
>>>>
>>>
>>> (think more little bit just now...)
>>>
>>>  Thread 1:                                           Thread 2 (fsync):
>>>   vfs_unlink                                          try_to_fix_pino
>>>     f2fs_unlink
>>>        f2fs_delete_entry
>>>          f2fs_drop_nlink  (i_sem, inode->i_nlink = 1)
>>>
>>>   (...   but this dentry still hashed)                  i_sem, check inode->i_nlink = 1
>>>                                                         i_sem d_find_alias
>>>
>>>   d_delete
>>>
>>> I'm not sure if fsync could still use some wrong alias by chance..
>>> completely untested, maybe just noise...

Another race condition could be:

Thread 1 (fsync)		Thread 2 (rename)
- f2fs_sync_fs
- try_to_fix_pino
				- f2fs_rename
				 - down_write
				 - file_lost_pino
				 - up_write
 - down_write
 - file_got_pino
 - up_write

Thanks,

>>>
>>
>> Right, good observation.  My patch makes it better, but it's still broken.
>>
>> I don't know how to fix it.  If we see i_nlink == 1 and multiple hashed
>> dentries, there doesn't appear to be a way to distingush which one corresponds
>> to the remaining link on-disk (if any; it may not even be in the dcache), and
>> which correspond to links that vfs_unlink() has deleted from disk but hasn't yet
>> done d_delete() on.
>>
>> One idea would be choose one, then take inode_lock_shared(dir) and do
>> __f2fs_find_entry() to check if the dentry is really still on-disk.  That's
>> heavyweight and error-prone though, and the locking could cause problems.
>>
>> I'm wondering though, does f2fs really need try_to_fix_pino() at all, and did it
>> ever really work?  It never actually updates the f2fs_inode::i_name to match the
>> new directory.  So independently of this bug with deleted links, I don't see how
>> it can possibly work as intended.
> 
> Part of my humble opinion would be "update pino in rename/unlink/link... such ops
> instead of in fsync" (maybe it makes better sense of locking)... But actually I'm
> not a f2fs folk now, just curious about what the original patch resolved with
> these new extra igrab/iput (as I said before, I could not find some clue previously).
> 
> Thanks,
> Gao Xiang
> 
>>
>> - Eric
> 
> 
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