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>

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;

fs/cifs/connect.c

@@ -63,7 +63,6 @@ extern mempool_t *cifs_req_poolp;
 #define TLINK_IDLE_EXPIRE	(600 * HZ)
 
 enum {
-
 	/* Mount options that take no arguments */
 	Opt_user_xattr, Opt_nouser_xattr,
 	Opt_forceuid, Opt_noforceuid,
@@ -95,7 +94,7 @@ enum {
 	Opt_cruid, Opt_gid, Opt_file_mode,
 	Opt_dirmode, Opt_port,
 	Opt_rsize, Opt_wsize, Opt_actimeo,
-	Opt_echo_interval,
+	Opt_echo_interval, Opt_max_credits,
 
 	/* Mount options which take string value */
 	Opt_user, Opt_pass, Opt_ip,
@@ -190,6 +189,7 @@ static const match_table_t cifs_mount_option_tokens = {
 	{ Opt_wsize, "wsize=%s" },
 	{ Opt_actimeo, "actimeo=%s" },
 	{ Opt_echo_interval, "echo_interval=%s" },
+	{ Opt_max_credits, "max_credits=%s" },
 
 	{ Opt_blank_user, "user=" },
 	{ Opt_blank_user, "username=" },
@@ -1586,6 +1586,15 @@ cifs_parse_mount_options(const char *mountdata, const char *devname,
 			}
 			vol->echo_interval = option;
 			break;
+		case Opt_max_credits:
+			if (get_option_ul(args, &option) || (option < 20) ||
+			    (option > 60000)) {
+				cifs_dbg(VFS, "%s: Invalid max_credits value\n",
+					 __func__);
+				goto cifs_parse_mount_err;
+			}
+			vol->max_credits = option;
+			break;
 
 		/* String Arguments */
 
@@ -3598,7 +3607,11 @@ try_mount_again:
 		bdi_destroy(&cifs_sb->bdi);
 		goto out;
 	}
-
+	if ((volume_info->max_credits < 20) ||
+	     (volume_info->max_credits > 60000))
+		server->max_credits = SMB2_MAX_CREDITS_AVAILABLE;
+	else
+		server->max_credits = volume_info->max_credits;
 	/* get a reference to a SMB session */
 	ses = cifs_get_smb_ses(server, volume_info);
 	if (IS_ERR(ses)) {

fs/cifs/smb2glob.h

@@ -61,14 +61,4 @@
 /* Maximum buffer size value we can send with 1 credit */
 #define SMB2_MAX_BUFFER_SIZE 65536
 
-/*
- * Maximum number of credits to keep available.
- * This value is chosen somewhat arbitrarily. The Windows client
- * defaults to 128 credits, the Windows server allows clients up to
- * 512 credits, and the NetApp server does not limit clients at all.
- * Choose a high enough value such that the client shouldn't limit
- * performance.
- */
-#define SMB2_MAX_CREDITS_AVAILABLE 32000
-
 #endif	/* _SMB2_GLOB_H */

fs/cifs/smb2ops.c

@@ -70,6 +70,10 @@ smb2_add_credits(struct TCP_Server_Info *server, const unsigned int add,
 	spin_lock(&server->req_lock);
 	val = server->ops->get_credits_field(server, optype);
 	*val += add;
+	if (*val > 65000) {
+		*val = 65000; /* Don't get near 64K credits, avoid srv bugs */
+		printk_once(KERN_WARNING "server overflowed SMB3 credits\n");
+	}
 	server->in_flight--;
 	if (server->in_flight == 0 && (optype & CIFS_OP_MASK) != CIFS_NEG_OP)
 		rc = change_conf(server);

fs/cifs/smb2pdu.c

@@ -105,11 +105,11 @@ smb2_hdr_assemble(struct smb2_hdr *hdr, __le16 smb2_cmd /* command */ ,
 
 		spin_lock(&server->req_lock);
 		/* Request up to 2 credits but don't go over the limit. */
-		if (server->credits >= SMB2_MAX_CREDITS_AVAILABLE)
+		if (server->credits >= server->max_credits)
 			hdr->CreditRequest = cpu_to_le16(0);
 		else
 			hdr->CreditRequest = cpu_to_le16(
-				min_t(int, SMB2_MAX_CREDITS_AVAILABLE -
+				min_t(int, server->max_credits -
 						server->credits, 2));
 		spin_unlock(&server->req_lock);
 	} else {