Re: Question on fscrypt_d_revalidate() and fstest generic/429

Eric Biggers
2017-05-16T22:07:15Z

On Tue, May 16, 2017 at 08:47:43AM +0200, Richard Weinberger wrote:
> Eric,
>
> Am 16.05.2017 um 01:25 schrieb Eric Biggers:
> > On Mon, May 15, 2017 at 09:51:03PM +0200, Richard Weinberger wrote:
> >>>
> >>> The test is repeatedly creating and removing a directory "dir" while lookups are
> >>> being done in it. It seems the problem is that many dentries are being created
> >>> for "dir", and they pin many different inodes, all at the same time. This
> >>> actually happens for ext4 too; it just doesn't cause an observable error.
> >>>
> >>> I doubt it's the right solution to make fscrypt_d_revalidate() look at
> >>> ->i_nlink, since ->d_revalidate() is meant to validate the filename, not the
> >>> inode. I think there is probably a VFS bug that is causing the dentries to not
> >>> be freed.
> >>
> >> Not sure. Al? :-)
> >>
> >
> > I can reproduce this on an unencrypted directory after updating path_init() in
> > fs/namei.c to always clear LOOKUP_RCU, so that all path lookups are done in
> > ref-walk mode. So I think fscrypt_d_revalidate() was only relevant because it
> > causes all path lookups to drop out of rcu-walk mode.
>
> On ext4 or UBIFS?
>
> Thanks,
> //richard

Both; the inode "leak" isn't filesystem-specific, beyond the fact that UBIFS
apparently limits the number of inodes on its orphan list while ext4 does not.
(How do I know it happens on ext4 then? /proc/slabinfo shows that lots of ext4
inodes have been allocated, and after an unclean shutdown and remounting, it's
reported that 50,000+ orphan inodes were deleted.)

There are lots of cases in which path lookups switch from rcu-walk mode into
ref-walk mode, so the fact that it was being caused by ->d_revalidate() in this
specific situation isn't really important, IMO.

I think the actual fix would be something along the lines of making vfs_rmdir()
unhash any negative child dentries, so that they get "killed" by dput() later.

Eric