Date: Fri, 7 Sep 2012 21:59:21 -0700 From: Gleb Kurtsou <gleb.kurtsou@gmail.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: pho@freebsd.org, fs@freebsd.org Subject: Re: Nullfs shared lookup Message-ID: <20120908045921.GA1419@reks> In-Reply-To: <20120905091854.GD33100@deviant.kiev.zoral.com.ua> References: <20120905091854.GD33100@deviant.kiev.zoral.com.ua>
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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?
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