Re: [PATCH] dcache: fix d_splice_alias handling of aliases

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Jan 15, 2014 at 06:34:56PM +0100, Miklos Szeredi wrote:
> On Wed, 2014-01-15 at 10:17 -0500, J. Bruce Fields wrote:
> > From: "J. Bruce Fields" <bfields@redhat.com>
> > 
> > d_splice_alias can create duplicate directory aliases (in the !new
> > case), or (in the new case) d_move without holding appropriate locks.
> 
> It can d_move, because the dentry is known to be disconnected, i.e. it
> doesn't have a parent for which we could obtain the lock.

DCACHE_DISCONNECTED doesn't mean that.

When you lookup a dentry by filehandle that dentry is initially marked
DCACHE_DISCONNECTED.  It is cleared only after reconnect_path() has
verified that the dentry is reachable all the way from the root.

So !DCACHE_DISCONNECTED implies that the dentry is connected all the way
up to the root, but the converse is not true.

This has been a source of confusion, but it is explained in
Documentation/filesystems/nfs/Exporting.  Recently I've cleaned up a few
odd uses of DCACHE_DISCONNECTED and rewritten reconnect_path(), partly
as an attempt to clarify the situation.

Let me know if any of that doesn't look right to you....

> > d_materialise_unique deals with both of these problems.  (The latter
> > seems to be dealt by trylocks (see __d_unalias), which look like they
> > could cause spurious lookup failures--but that's at least better than
> > corrupting the dcache.)
> > 
> > Signed-off-by: J. Bruce Fields <bfields@redhat.com>
> > ---
> >  fs/dcache.c |   25 +------------------------
> >  1 file changed, 1 insertion(+), 24 deletions(-)
> > 
> > Only lightly tested....  If this is right, then we can also just ditch
> > d_splice_alias completely, and clean up the various d_find_alias's.
> > 
> > I think the only reason we have both d_splice_alias and
> > d_materialise_unique is that the former was written for exportable
> > filesystems and the latter for distributed filesystems.
> > 
> > But we have at least one exportable filesystem (fuse) using
> > d_materialise_unique.  And I doubt d_splice_alias was ever completely
> > correct even for on-disk filesystems.
> > 
> > Am I missing some subtlety?
> 
> One subtle difference is that for a non-directory d_splice_alias() will
> reconnect a DCACHE_DISCONNECTED dentry if one exists, while
> d_materialise_unique() will not.

Actually until f80de2cde10350b8d146e375ff8b634e72e6a827 "dcache: don't
clear DCACHE_DISCONNECTED too early", it was the reverse:
d_materialise_unique cleared DISCONNECTED and d_splice_alias (correctly)
did not.

The only place where it should be cleared is reconnect_path().

> Does this matter in practice?   The small number of extra dentries
> probably does not matter.

Directories are assumed to have unique aliases.  When they don't, the
kernel can deadlock or crash.

--b.