9p: implement optional loose read cache

While cacheing is generally frowned upon in the 9p world, it has its
place -- particularly in situations where the remote file system is
exclusive and/or read-only.  The vacfs views of venti content addressable
store are a real-world instance of such a situation.  To facilitate higher
performance for these workloads (and eventually use the fscache patches),
we have enabled a "loose" cache mode which does not attempt to maintain
any form of consistency on the page-cache or dcache.  This results in over
two orders of magnitude performance improvement for cacheable block reads
in the Bonnie benchmark.  The more aggressive use of the dcache also seems
to improve metadata operational performance.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
index 062daa6000ab..ddffd8aa902d 100644
--- a/fs/9p/vfs_dentry.c
+++ b/fs/9p/vfs_dentry.c
@@ -53,10 +53,31 @@
 static int v9fs_dentry_delete(struct dentry *dentry)
 {
         dprintk(DEBUG_VFS, " dentry: %s (%p)\n", dentry->d_iname, dentry);
+
         return 1;
 }
 
 /**
+ * v9fs_cached_dentry_delete - called when dentry refcount equals 0
+ * @dentry:  dentry in question
+ *
+ * Only return 1 if our inode is invalid.  Only non-synthetic files
+ * (ones without mtime == 0) should be calling this function.
+ *
+ */
+
+static int v9fs_cached_dentry_delete(struct dentry *dentry)
+{
+        struct inode *inode = dentry->d_inode;
+        dprintk(DEBUG_VFS, " dentry: %s (%p)\n", dentry->d_iname, dentry);
+
+        if(!inode)
+                return 1;
+
+        return 0;
+}
+
+/**
  * v9fs_dentry_release - called when dentry is going to be freed
  * @dentry:  dentry that is being release
  *
@@ -87,6 +108,11 @@ void v9fs_dentry_release(struct dentry *dentry)
         }
 }
 
+struct dentry_operations v9fs_cached_dentry_operations = {
+        .d_delete = v9fs_cached_dentry_delete,
+        .d_release = v9fs_dentry_release,
+};
+
 struct dentry_operations v9fs_dentry_operations = {
         .d_delete = v9fs_dentry_delete,
         .d_release = v9fs_dentry_release,