SMB3: Add mount parameter to allow user to override max credits

Add mount option "max_credits" to allow setting maximum SMB3
credits to any value from 10 to 64000 (default is 32000).
This can be useful to workaround servers with problems allocating
credits, or to throttle the client to use smaller amount of
simultaneous i/o or to workaround server performance issues.

Also adds a cap, so that even if the server granted us more than
65000 credits due to a server bug, we would not use that many.

Signed-off-by: Steve French <steve.french@primarydata.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
index 65f78b7a9062..cd80594f14d0 100644
--- a/fs/cifs/cifsglob.h
+++ b/fs/cifs/cifsglob.h
@@ -75,6 +75,18 @@
 #define SMB_ECHO_INTERVAL_MAX 600
 #define SMB_ECHO_INTERVAL_DEFAULT 60
 
+/*
+ * Default number of credits to keep available for SMB3.
+ * This value is chosen somewhat arbitrarily. The Windows client
+ * defaults to 128 credits, the Windows server allows clients up to
+ * 512 credits (or 8K for later versions), and the NetApp server
+ * does not limit clients at all.  Choose a high enough default value
+ * such that the client shouldn't limit performance, but allow mount
+ * to override (until you approach 64K, where we limit credits to 65000
+ * to reduce possibility of seeing more server credit overflow bugs.
+ */
+#define SMB2_MAX_CREDITS_AVAILABLE 32000
+
 #include "cifspdu.h"
 
 #ifndef XATTR_DOS_ATTRIB
@@ -510,6 +522,7 @@ struct smb_vol {
 	struct sockaddr_storage srcaddr; /* allow binding to a local IP */
 	struct nls_table *local_nls;
 	unsigned int echo_interval; /* echo interval in secs */
+	unsigned int max_credits; /* smb3 max_credits 10 < credits < 60000 */
 };
 
 #define CIFS_MOUNT_MASK (CIFS_MOUNT_NO_PERM | CIFS_MOUNT_SET_UID | \
@@ -567,7 +580,8 @@ struct TCP_Server_Info {
 	bool noblocksnd;		/* use blocking sendmsg */
 	bool noautotune;		/* do not autotune send buf sizes */
 	bool tcp_nodelay;
-	int credits;  /* send no more requests at once */
+	unsigned int credits;  /* send no more requests at once */
+	unsigned int max_credits; /* can override large 32000 default at mnt */
 	unsigned int in_flight;  /* number of requests on the wire to server */
 	spinlock_t req_lock;  /* protect the two values above */
 	struct mutex srv_mutex;