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

[Chao Yu <yuchao0@huawei.com>]

On 2020/5/6 14:55, Chao Yu wrote:
> On 2020/5/6 9:24, 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.
>>
>>>
>>>>
>>>> We need to find the dentry whose parent directory is still exists, i.e. the
>>>> parent directory that is counting towards 'inode->i_nlink == 1'.
>>>
>>> directory counting towards 'inode->i_nlink == 1', what's happening?
>>
>> The non-deleted link is the one counted in i_nlink.
>>
>>>
>>>>
>>>> I think d_find_alias() is what we're looking for.
>>>
>>> It may be simply dentry->d_parent (stable/positive as you said before, and it's
>>> not empty). why need to d_find_alias()?
>>
>> Because we need to get the dentry that hasn't been deleted yet, which isn't
>> necessarily the one associated with the file descriptor being fsync()'ed.
>>
>>> And what is the original problem? I could not get some clue from the original
>>> patch description (I only saw some extra igrab/iput because of some unknown
>>> reasons), it there some backtrace related to the problem?
>>
>> The problem is that i_pino gets set incorrectly.  I just noticed this while
>> reviewing the code.  It's not hard to reproduce, e.g.:
>>
>> #include <unistd.h>
>> #include <fcntl.h>
>> #include <sys/stat.h>
>>
>> int main()
>> {
>>         int fd;
>>
>>         mkdir("dir1", 0700);
>>         mkdir("dir2", 0700);
>>         mknod("dir1/file", S_IFREG|0600, 0);
>>         link("dir1/file", "dir2/file");
>>         fd = open("dir2/file", O_WRONLY);
>>         unlink("dir2/file");
>>         write(fd, "X", 1);
>>         fsync(fd);
>> }
>>
>> Then:
>>
>> sync
>> echo N | dump.f2fs -i $(stat -c %i dir1/file) /dev/vdb | grep 'i_pino'
>> echo "dir1 (correct): $(stat -c %i dir1)"
>> echo "dir2 (wrong): $(stat -c %i dir2)"
>>
>> i_pino will point to dir2 rather than dir1 as expected.
> 
> Could you add above testcase into commit message of your patch? it will
> be easier to understand the issue we solved with it.
> 
> In addition, how about adding this testcase in fstest as a generic one?

Oops, it's not a generic one... please ignore this.

> 
>>
>> - Eric
>>
>>
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>>
> 
> 
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