cifs: client: allow changing multichannel mount options on remount

Previously, the client did not update a session's channel state when
multichannel or max_channels mount options were changed via remount.
This led to inconsistent behavior and prevented enabling or disabling
multichannel support without a full unmount/remount cycle.

Enable dynamic reconfiguration of multichannel and max_channels during
remount by:
- Introducing smb3_sync_ses_chan_max(), a centralized function for
  channel updates which synchronizes the session's channels with the
  updated configuration.
- Replacing cifs_disable_secondary_channels() with
  cifs_decrease_secondary_channels(), which accepts a disable_mchan
  flag to support multichannel disable when the server stops supporting
  multichannel.
- Updating remount logic to detect changes in multichannel or
  max_channels and trigger appropriate session/channel updates.

Current limitation:
- The query_interfaces worker runs even when max_channels=1 so that
  multichannel can be enabled later via remount without requiring an
  unmount. This is a temporary approach and may be refined in the
  future.

Users can safely modify multichannel and max_channels on an existing
mount. The client will correctly adjust the session's channel state to
match the new configuration, preserving durability where possible and
avoiding unnecessary disconnects.

Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Rajasi Mandal <rajasimandal@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/cifsproto.h

@@ -635,6 +635,8 @@ int cifs_alloc_hash(const char *name, struct shash_desc **sdesc);
 void cifs_free_hash(struct shash_desc **sdesc);
 
 int cifs_try_adding_channels(struct cifs_ses *ses);
+int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
+					bool from_reconnect, bool disable_mchan);
 bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
 void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
 
@@ -660,7 +662,7 @@ bool
 cifs_chan_is_iface_active(struct cifs_ses *ses,
 			  struct TCP_Server_Info *server);
 void
-cifs_disable_secondary_channels(struct cifs_ses *ses);
+cifs_decrease_secondary_channels(struct cifs_ses *ses, bool disable_mchan);
 void
 cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
 int

fs/smb/client/connect.c

@@ -3926,7 +3926,9 @@ int cifs_mount(struct cifs_sb_info *cifs_sb, struct smb3_fs_context *ctx)
 	ctx->prepath = NULL;
 
 out:
-	cifs_try_adding_channels(mnt_ctx.ses);
+	smb3_update_ses_channels(mnt_ctx.ses, mnt_ctx.server,
+				  false /* from_reconnect */,
+				  false /* disable_mchan */);
 	rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
 	if (rc)
 		goto error;

fs/smb/client/fs_context.c

@@ -758,6 +758,7 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
 static int smb3_fs_context_parse_monolithic(struct fs_context *fc,
 					    void *data);
 static int smb3_get_tree(struct fs_context *fc);
+static void smb3_sync_ses_chan_max(struct cifs_ses *ses, unsigned int max_channels);
 static int smb3_reconfigure(struct fs_context *fc);
 
 static const struct fs_context_operations smb3_fs_context_ops = {
@@ -1055,6 +1056,22 @@ int smb3_sync_session_ctx_passwords(struct cifs_sb_info *cifs_sb, struct cifs_se
 	return 0;
 }
 
+/*
+ * smb3_sync_ses_chan_max - Synchronize the session's maximum channel count
+ * @ses: pointer to the old CIFS session structure
+ * @max_channels: new maximum number of channels to allow
+ *
+ * Updates the session's chan_max field to the new value, protecting the update
+ * with the session's channel lock. This should be called whenever the maximum
+ * allowed channels for a session changes (e.g., after a remount or reconfigure).
+ */
+static void smb3_sync_ses_chan_max(struct cifs_ses *ses, unsigned int max_channels)
+{
+	spin_lock(&ses->chan_lock);
+	ses->chan_max = max_channels;
+	spin_unlock(&ses->chan_lock);
+}
+
 static int smb3_reconfigure(struct fs_context *fc)
 {
 	struct smb3_fs_context *ctx = smb3_fc2context(fc);
@@ -1137,7 +1154,39 @@ static int smb3_reconfigure(struct fs_context *fc)
 		ses->password2 = new_password2;
 	}
 
+	/*
+	 * If multichannel or max_channels has changed, update the session's channels accordingly.
+	 * This may add or remove channels to match the new configuration.
+	 */
+	if ((ctx->multichannel != cifs_sb->ctx->multichannel) ||
+	    (ctx->max_channels != cifs_sb->ctx->max_channels)) {
+
+		/* Synchronize ses->chan_max with the new mount context */
+		smb3_sync_ses_chan_max(ses, ctx->max_channels);
+		/* Now update the session's channels to match the new configuration */
+		/* Prevent concurrent scaling operations */
+		spin_lock(&ses->ses_lock);
+		if (ses->flags & CIFS_SES_FLAG_SCALE_CHANNELS) {
+			spin_unlock(&ses->ses_lock);
 			mutex_unlock(&ses->session_mutex);
+			return -EINVAL;
+		}
+		ses->flags |= CIFS_SES_FLAG_SCALE_CHANNELS;
+		spin_unlock(&ses->ses_lock);
+
+		mutex_unlock(&ses->session_mutex);
+
+		rc = smb3_update_ses_channels(ses, ses->server,
+					       false /* from_reconnect */,
+					       false /* disable_mchan */);
+
+		/* Clear scaling flag after operation */
+		spin_lock(&ses->ses_lock);
+		ses->flags &= ~CIFS_SES_FLAG_SCALE_CHANNELS;
+		spin_unlock(&ses->ses_lock);
+	} else {
+		mutex_unlock(&ses->session_mutex);
+	}
 
 	STEAL_STRING(cifs_sb, ctx, domainname);
 	STEAL_STRING(cifs_sb, ctx, nodename);

fs/smb/client/sess.c

@@ -265,12 +265,16 @@ int cifs_try_adding_channels(struct cifs_ses *ses)
 }
 
 /*
- * called when multichannel is disabled by the server.
+ * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
- * this always gets called from smb2_reconnect
+ * @ses: pointer to the CIFS session structure
- * and cannot get called in parallel threads.
+ * @disable_mchan: if true, reduce to a single channel; if false, reduce to chan_max
+ *
+ * This function disables and cleans up extra secondary channels for a CIFS session.
+ * If called during reconfiguration, it reduces the channel count to the new maximum (chan_max).
+ * Otherwise, it disables all but the primary channel.
  */
 void
-cifs_disable_secondary_channels(struct cifs_ses *ses)
+cifs_decrease_secondary_channels(struct cifs_ses *ses, bool disable_mchan)
 {
 	int i, chan_count;
 	struct TCP_Server_Info *server;
@@ -281,12 +285,16 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
 	if (chan_count == 1)
 		goto done;
 
+	/* Update the chan_count to the new maximum */
+	if (disable_mchan) {
+		cifs_dbg(FYI, "server does not support multichannel anymore.\n");
 		ses->chan_count = 1;
+	} else {
+		ses->chan_count = ses->chan_max;
+	}
 
-	/* for all secondary channels reset the need reconnect bit */
+	/* Disable all secondary channels beyond the new chan_count */
-	ses->chans_need_reconnect &= 1;
+	for (i = ses->chan_count ; i < chan_count; i++) {
-
-	for (i = 1; i < chan_count; i++) {
 		iface = ses->chans[i].iface;
 		server = ses->chans[i].server;
 
@@ -318,6 +326,15 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
 		spin_lock(&ses->chan_lock);
 	}
 
+	/* For extra secondary channels, reset the need reconnect bit */
+	if (ses->chan_count == 1) {
+		cifs_dbg(VFS, "Disable all secondary channels\n");
+		ses->chans_need_reconnect &= 1;
+	} else {
+		cifs_dbg(VFS, "Disable extra secondary channels\n");
+		ses->chans_need_reconnect &= ((1UL << ses->chan_max) - 1);
+	}
+
 done:
 	spin_unlock(&ses->chan_lock);
 }

fs/smb/client/smb2pdu.c

@@ -168,7 +168,7 @@ smb2_hdr_assemble(struct smb2_hdr *shdr, __le16 smb2_cmd,
 static int
 cifs_chan_skip_or_disable(struct cifs_ses *ses,
 			  struct TCP_Server_Info *server,
-			  bool from_reconnect)
+			  bool from_reconnect, bool disable_mchan)
 {
 	struct TCP_Server_Info *pserver;
 	unsigned int chan_index;
@@ -206,14 +206,46 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
 		return -EHOSTDOWN;
 	}
 
-	cifs_server_dbg(VFS,
+	cifs_decrease_secondary_channels(ses, disable_mchan);
-		"server does not support multichannel anymore. Disable all other channels\n");
-	cifs_disable_secondary_channels(ses);
-
 
 	return 0;
 }
 
+/*
+ * smb3_update_ses_channels - Synchronize session channels with new configuration
+ * @ses: pointer to the CIFS session structure
+ * @server: pointer to the TCP server info structure
+ * @from_reconnect: indicates if called from reconnect context
+ * @disable_mchan: indicates if called from reconnect to disable multichannel
+ *
+ * Returns 0 on success or error code on failure.
+ *
+ * Outside of reconfigure, this function is called from cifs_mount() during mount
+ * and from reconnect scenarios to adjust channel count when the
+ * server's multichannel support changes.
+ */
+int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
+			bool from_reconnect, bool disable_mchan)
+{
+	int rc = 0;
+	/*
+	 * Manage session channels based on current count vs max:
+	 * - If disable requested, skip or disable the channel
+	 * - If below max channels, attempt to add more
+	 * - If above max channels, skip or disable excess channels
+	 */
+	if (disable_mchan)
+		rc = cifs_chan_skip_or_disable(ses, server, from_reconnect, disable_mchan);
+	else {
+		if (ses->chan_count < ses->chan_max)
+			rc = cifs_try_adding_channels(ses);
+		else if (ses->chan_count > ses->chan_max)
+			rc = cifs_chan_skip_or_disable(ses, server, from_reconnect, disable_mchan);
+	}
+
+	return rc;
+}
+
 static int
 smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 	       struct TCP_Server_Info *server, bool from_reconnect)
@@ -355,8 +387,8 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 	 */
 	if (ses->chan_count > 1 &&
 	    !(server->capabilities & SMB2_GLOBAL_CAP_MULTI_CHANNEL)) {
-		rc = cifs_chan_skip_or_disable(ses, server,
+		rc = smb3_update_ses_channels(ses, server,
-					       from_reconnect);
+					       from_reconnect, true /* disable_mchan */);
 		if (rc) {
 			mutex_unlock(&ses->session_mutex);
 			goto out;
@@ -438,8 +470,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 			 * treat this as server not supporting multichannel
 			 */
 
-			rc = cifs_chan_skip_or_disable(ses, server,
+			rc = smb3_update_ses_channels(ses, server,
-						       from_reconnect);
+						       from_reconnect,
+						       true /* disable_mchan */);
 			goto skip_add_channels;
 		} else if (rc)
 			cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
@@ -451,7 +484,8 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 			if (ses->chan_count == 1)
 				cifs_server_dbg(VFS, "supports multichannel now\n");
 
-			cifs_try_adding_channels(ses);
+			smb3_update_ses_channels(ses, server, from_reconnect,
+						  false /* disable_mchan */);
 		}
 	} else {
 		mutex_unlock(&ses->session_mutex);
@@ -1105,7 +1139,6 @@ SMB2_negotiate(const unsigned int xid,
 		req->SecurityMode = 0;
 
 	req->Capabilities = cpu_to_le32(server->vals->req_capabilities);
-	if (ses->chan_max > 1)
 	req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	/* ClientGUID must be zero for SMB2.02 dialect */
@@ -1332,7 +1365,6 @@ int smb3_validate_negotiate(const unsigned int xid, struct cifs_tcon *tcon)
 
 	pneg_inbuf->Capabilities =
 			cpu_to_le32(server->vals->req_capabilities);
-	if (tcon->ses->chan_max > 1)
 	pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	memcpy(pneg_inbuf->Guid, server->client_guid,