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

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Date:   Wed, 6 May 2020 14:47:19 +0800
From:   Gao Xiang <hsiangkao@....com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino

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...

Thanks,
Gao Xiang