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Subject[patch 3/3] Support removal of unused dentry entries via SLUB defrag interface
This patch allows the removal of unused dentry entries in a partial
populated slab page. Very limited (yet) in what it can do for reclaim
but this catches bad cases in which we have a long list of partial
slabs with a few entries in each of them. We can free up the slabs
that have only unused dentry entries in them.

get_dentry() uses the dcache lock and then works with dget_locked
to obtain a reference to the dentry. An additional complication is that
the dentry may be in process of being freed or it may just have been
allocated. In that case d_inode is NULL. If we discover this then we
simply stay away from the object and return 1 to indicate to the
defrag logic that this object will be free. Otherwise we increment
the refcount and return success.

kick_dentry() is called after get_dentry_reference() has
been used and after the slab has dropped all of its own locks. The dentry
pruning for unused entries works in a straighforward way.

Note: The code here could be significantly improved. If we could
get to a point where all used dentries could be moved then full
dentry slab defragmentation and vacating of dentry slab pages would
become possible.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
fs/dcache.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 81 insertions(+), 8 deletions(-)

Index: slub/fs/dcache.c
===================================================================
--- slub.orig/fs/dcache.c 2007-05-16 20:58:02.000000000 -0700
+++ slub/fs/dcache.c 2007-05-16 20:59:27.000000000 -0700
@@ -2114,18 +2114,91 @@ static void __init dcache_init_early(voi
INIT_HLIST_HEAD(&dentry_hashtable[loop]);
}

+/*
+ * The slab is holding off frees. Thus we can safely examine
+ * the object without the danger of it vanishing from under us.
+ */
+static int get_dentry(struct kmem_cache *s, void *private)
+{
+ struct dentry *dentry = private;
+ int result = 0;
+
+ spin_lock(&dcache_lock);
+ /*
+ * dentry->d_inode is set to NULL when the dentry
+ * is freed. Use that as an indicator that we should
+ * not interfere with the freeing process.
+ */
+ if (dentry->d_inode) {
+ dget_locked(dentry);
+ if (atomic_read(&dentry->d_count) > 2)
+ /*
+ * Moving of dentries in use not
+ * implemented yet.
+ */
+ result = -EINVAL;
+ } else
+ result = 1;
+ spin_unlock(&dcache_lock);
+ return result;
+}
+
+static void put_dentry(struct kmem_cache *s, void *private)
+{
+ struct dentry *dentry = private;
+
+ dput(dentry);
+}
+
+/*
+ * Slab has dropped all the locks. Get rid of the
+ * refcount we obtained earlier and also rid of the
+ * object.
+ */
+static int kick_dentry(struct kmem_cache *s, void *private)
+{
+ struct dentry *dentry = private;
+
+ spin_lock(&dcache_lock);
+ spin_lock(&dentry->d_lock);
+ if (atomic_read(&dentry->d_count) > 1) {
+ /*
+ * Reference count was increased.
+ * We need to abandon the freeing of
+ * objects.
+ */
+ spin_unlock(&dentry->d_lock);
+ spin_unlock(&dcache_lock);
+ dput(dentry);
+ return -EBUSY;
+ }
+
+ /* Remove from LRU */
+ if (!list_empty(&dentry->d_lru)) {
+ dentry_stat.nr_unused--;
+ list_del_init(&dentry->d_lru);
+ }
+ /* Drop the entry */
+ prune_one_dentry(dentry, 1);
+ spin_unlock(&dcache_lock);
+ return 0;
+}
+
+static struct kmem_cache_ops dentry_kmem_cache_ops = {
+ .get = get_dentry,
+ .put = put_dentry,
+ .kick = kick_dentry,
+ .sync = synchronize_rcu
+};
+
static void __init dcache_init(unsigned long mempages)
{
int loop;

- /*
- * A constructor could be added for stable state like the lists,
- * but it is probably not worth it because of the cache nature
- * of the dcache.
- */
- dentry_cache = KMEM_CACHE(dentry,
- SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD);
-
+ dentry_cache = KMEM_CACHE_OPS(dentry,
+ SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_MEM_SPREAD,
+ &dentry_kmem_cache_ops);
+
register_shrinker(&dcache_shrinker);

/* Hash may have been set up in dcache_init_early */
--
-
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