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

[Eric Biggers <ebiggers@kernel.org>]

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...
> 

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.

- Eric


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