On Wed 30-05-12 13:37:09, J. Bruce Fields wrote: > On Tue, May 29, 2012 at 10:08:56PM +0200, Jan Kara wrote: > > On Tue 29-05-12 21:50:19, Jan Kara wrote: > > > On Mon 28-05-12 17:05:11, Ted Tso wrote: > > > > On Mon, May 28, 2012 at 02:29:05PM -0600, Andreas Dilger wrote: > > > > > This patch is good from the POV of covering all filesystems, and > > > > > avoiding the deadlock at the dcache level. It would be possible to > > > > > detect this problem in the filesystem itself during lookup, before > > > > > the bad link got into the dcache itself. Something like: > > > > > > > > I like that as a solution for detecting the problem in ext4. As you > > > > say, it's still an issue for other file systems, and so the patch I > > > > proposed is still probably a good idea for the VFS. But this way ext4 > > > > (and ext3 when Jan backports it) will be able to detect the problem > > > > and mark the file system as being corrupted. > > > Actually, I think there's even better way. d_splice_alias() can rather > > > easily detect the problem and report it to filesystem. The advantage is > > > that the check in d_splice_alias() can catch any "hardlinks" to > > > directories, not just self loops. The patch is attached, I also have > > > corresponding handling written for ext? filesystems but that's trivial. > > > I'll post the whole series to Al to have a look. > > And now with the attachment. Sorry. > > Well, my understanding of d_splice_alias is that it should just return > the existing dentry instead of failing. (It does that now for > DISCONNECTED dentries, but I don't understand why they're special.) > So that's what: > > http://git.kernel.org/?p=linux/kernel/git/viro/vfs.git;a=commit;h=9d345b3217b384813680901d42eae3fb380b9f77 > > does. Thanks for the pointer. In the case I tried to solve, returning the existing dentry will solve the deadlocks, just user won't be warned that the filesystem is corrupted. Since you seem to describe a valid case where we can spot other !DISCONNECTED dentry of a directory, I guess we have no other choice than using your approach. We could do some sanity checks in ->lookup method (like Andreas suggested) but they are not that powerful as a check in d_splice_alias() can be. But what can one do... Honza > > >From 0715b656ac88ce1bb62800b14d99ef2e25c26d28 Mon Sep 17 00:00:00 2001 > > From: Jan Kara <jack@xxxxxxx> > > Date: Tue, 29 May 2012 21:19:01 +0200 > > Subject: [PATCH 1/4] vfs: Avoid creation of directory loops for corrupted filesystems > > > > When a directory hierarchy is corrupted (e. g. due to a bit flip on the media), > > it can happen that it contains loops of directories. That creates possibilities > > for deadlock when locking directories. > > > > Fix the problem by checking in d_splice_alias() that when we splice a > > directory, it does not have any other connected alias. > > > > Reported-by: Sami Liedes <sami.liedes@xxxxxx> > > Signed-off-by: Jan Kara <jack@xxxxxxx> > > --- > > fs/dcache.c | 4 ++++ > > 1 files changed, 4 insertions(+), 0 deletions(-) > > > > diff --git a/fs/dcache.c b/fs/dcache.c > > index 4435d8b..ca31a1e 100644 > > --- a/fs/dcache.c > > +++ b/fs/dcache.c > > @@ -1658,6 +1658,10 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry) > > d_move(new, dentry); > > iput(inode); > > } else { > > + if (unlikely(!list_empty(&inode->i_dentry))) { > > + spin_unlock(&inode->i_lock); > > + return ERR_PTR(-EIO); > > + } > > /* already taking inode->i_lock, so d_add() by hand */ > > __d_instantiate(dentry, inode); > > spin_unlock(&inode->i_lock); > > -- > > 1.7.1 > > > -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
