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, 8 insertions, 1 deletions

diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h
index c134d104cb28..820bf5ca35d8 100644
--- a/fs/9p/v9fs.h
+++ b/fs/9p/v9fs.h
@@ -47,7 +47,7 @@ struct v9fs_session_info {
         unsigned int afid;      /* authentication fid */
         unsigned int rfdno;     /* read file descriptor number */
         unsigned int wfdno;     /* write file descriptor number */
-
+        unsigned int cache;     /* cache mode */
 
         char *name;             /* user name to mount as */
         char *remotename;       /* name of remote hierarchy being mounted */
@@ -73,6 +73,13 @@ enum {
         PROTO_FD,
 };
 
+/* possible values of ->cache */
+/* eventually support loose, tight, time, session, default always none */
+enum {
+        CACHE_NONE,             /* default */
+        CACHE_LOOSE,            /* no consistency */
+};
+
 extern struct dentry *v9fs_debugfs_root;
 
 int v9fs_session_init(struct v9fs_session_info *, const char *, char *);