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Date:   Thu, 7 May 2020 06:36:26 +0800
From:   Gao Xiang <hsiangkao@gmx.com>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Jaegeuk Kim <jaegeuk@kernel.org>, kernel-team@android.com,
        linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: get parent inode when recovering pino
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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 know=
ledge?
> > > > >  inode itself is locked when fsyncing, so
> > > > >
> > > > >    if the fsync inode->i_nlink =3D=3D 1, this inode has only one=
 hard link
> > > > >    (not deleted yet) and should belong to a single directory; an=
d
> > > > >
> > > > >    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 scena=
rio?
> > > > > Thanks a lot!
> > > >
> > > > i_nlink =3D=3D 1 just means that there is one non-deleted link.  T=
here can be links
> > > > that have since been deleted, and file descriptors can still be op=
en 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 =3D 1)
> >
> >   (...   but this dentry still hashed)                  i_sem, check i=
node->i_nlink =3D 1
> >                                                         i_sem d_find_a=
lias
> >
> >   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 broke=
n.
>
> I don't know how to fix it.  If we see i_nlink =3D=3D 1 and multiple has=
hed
> dentries, there doesn't appear to be a way to distingush which one corre=
sponds
> 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 ha=
sn'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.  Tha=
t'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, an=
d did it
> ever really work?  It never actually updates the f2fs_inode::i_name to m=
atch 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... s=
uch ops
instead of in fsync" (maybe it makes better sense of locking)... But actua=
lly I'm
not a f2fs folk now, just curious about what the original patch resolved w=
ith
these new extra igrab/iput (as I said before, I could not find some clue p=
reviously).

Thanks,
Gao Xiang

>
> - Eric