Nullfs shared lookup

[Gleb Kurtsou]

On (05/09/2012 12:18), Konstantin Belousov wrote:
> I, together with Peter Holm, developed a patch to enable shared lookups
> on nullfs mounts when lower filesystem allows the shared lookups. The lack
> of shared lookup support for nullfs is quite visible on any VFS-intensive
> workloads which utilize path translations. In particular, it was a complain
> on $dayjob which started me thinking about this issue.
> 
> There are two problems which prevent direct translation of shared
> lookup bit into nullfs upper mount bit:
> 
> 1. When vfs_lookup() calls VOP_LOOKUP() for nullfs, which passes lookup
> operation to lower fs, resulting vnode is often only shared-locked. Then
> null_nodeget() cannot instantiate covering vnode for lower vnode, since
> insmntque1() and null_hashins() require exclusive lock on the lower.
> 
> The solution is straightforward, if null hash failed to find pre-existing
> nullfs vnode for lower vnode, the lower vnode lock is upgraded.
> 
> 2. (More serious). Nullfs reclaims its vnodes on deactivation. The cause
> is due to nullfs inability to detect reclamation of the lower vnode.
> Reclamation of a nullfs vnode at deactivation time prevents a reference
> to the lower vnode to become stale.
> 
> Unfortunately, this means that all lookups on nullfs need exclusive lock
> to instantiate upper vnode, which is never cached.
> 
> Solution which we propose is to add VFS notification to the upper
> filesystem about reclamation of the vnode in the lower filesystem. Now,
> vgone() calls new VFS op vfs_reclaim_lowervp() with an argument lowervp
> which is reclaimed. It is possible to register several reclamation event
> listeners, to correctly handle the case of several nullfs mounts over
> the same directory.
> 
> For the filesystem not having nullfs mounts over it, the overhead added is
> a single mount interlock lock/unlock in the vnode reclamation path.
> 
> Benchmarks consisting of up 1K threads doing parallel stat(2) on the
> same file demonstate almost constant execution time, not depending of
> number of running threads. While without the patch, exec time between
> single-threaded run and run with 1024 threads performing the same total
> count of stat(2), differ in 6 times.
> 
> Somewhat problematic detail, IMO, is that nullfs reclamation procedure
> calls vput() on the lowervp vnode, temporary unlocking the vnode being
> reclaimed. This seems to be fine for MPSAFE filesystems, but not-MPSAFE
> code often put partially initialized vnode on some globally visible
> list, and later can decide that half-constructed vnode is not needed.
> If nullfs mount is created above such filesystem, then other threads
> might catch such not properly initialized vnode. Instead of trying
> to overcome this case, e.g. by recursing the lower vnode lock in
> null_reclaim_lowervp(), I decided to rely on nearby extermination of
> non-MPSAFE filesystems support.
> 
> I think that unionfs can also benefit from this mechanism, but I did not
> even looked at unionfs.
> 
> Patch is available at
> http://people.freebsd.org/~kib/misc/nullfs_shared_lookup.1.patch
> It survived stress2 torturing.
> 
> Comments ?

I only had a glance look at the patch, sorry it I missed something
obvious.  How do we achieve propagation of rename/rm/rmdir to upper
level name cache?