[PATCH] cifs: client: allow changing multichannel mount options on remount

[PATCH] cifs: client: allow changing multichannel mount options on remount

[Rajasi Mandal]

From: Rajasi Mandal <rajasimandal@microsoft.com>

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 from_reconfigure
  flag to support targeted cleanup during reconfiguration.
- 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>
---
 fs/smb/client/cifsproto.h  |  4 ++-
 fs/smb/client/connect.c    |  4 ++-
 fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
 fs/smb/client/sess.c       | 32 +++++++++++++++-------
 fs/smb/client/smb2pdu.c    | 55 ++++++++++++++++++++++++++++----------
 5 files changed, 119 insertions(+), 26 deletions(-)

diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
index 3528c365a452..a1fc9e1918bc 100644
--- a/fs/smb/client/cifsproto.h
+++ b/fs/smb/client/cifsproto.h
@@ -649,6 +649,8 @@ 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 from_reconfigure);
 bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
 void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
 
@@ -674,7 +676,7 @@ 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 from_reconfigure);
 void
 cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
 int
diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index 55cb4b0cbd48..cebe4a5f54f2 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
 	rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
 	if (rc)
 		goto error;
diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 0f894d09157b..751ed6ebd458 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -717,6 +717,7 @@ 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 = {
@@ -1013,6 +1014,22 @@ 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);
@@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
 		ses->password2 = new_password2;
 	}
 
-	mutex_unlock(&ses->session_mutex);
+	/*
+	 * 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);
+			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 */,
+					       true /* from_reconfigure */);
+
+		/* 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);
diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index ef3b498b0a02..cfd83986a84a 100644
--- a/fs/smb/client/sess.c
+++ b/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.
- * this always gets called from smb2_reconnect
- * and cannot get called in parallel threads.
+ * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
+ * @ses: pointer to the CIFS session structure
+ * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
+ *
+ * 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 from_reconfigure)
 {
 	int i, chan_count;
 	struct TCP_Server_Info *server;
@@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
 	if (chan_count == 1)
 		goto done;
 
-	ses->chan_count = 1;
-
-	/* for all secondary channels reset the need reconnect bit */
-	ses->chans_need_reconnect &= 1;
+	/* Update the chan_count to the new maximum */
+	if (from_reconfigure)
+		ses->chan_count = ses->chan_max;
+	else
+		ses->chan_count = 1;
 
-	for (i = 1; i < chan_count; i++) {
+	for (i = ses->chan_max ; i < chan_count; i++) {
 		iface = ses->chans[i].iface;
 		server = ses->chans[i].server;
 
@@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
 }
diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 8b4a4573e9c3..d051da46eaab 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/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 from_reconfigure)
 {
 	struct TCP_Server_Info *pserver;
 	unsigned int chan_index;
@@ -206,14 +206,41 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
 		return -EHOSTDOWN;
 	}
 
-	cifs_server_dbg(VFS,
-		"server does not support multichannel anymore. Disable all other channels\n");
-	cifs_disable_secondary_channels(ses);
-
+	cifs_decrease_secondary_channels(ses, from_reconfigure);
 
 	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
+ * @from_reconfigure: indicates if called from reconfigure context
+ *
+ * Returns 0 on success or error code on failure.
+ *
+ * Note: Outside of reconfigure, this function is only called from reconnect scenarios
+ * when the server stops advertising multichannel (MC) capability.
+ */
+int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
+			bool from_reconnect, bool from_reconfigure)
+{
+	int rc = 0;
+	/*
+	 * If the current channel count is less than the new chan_max,
+	 * try to add channels to reach the new maximum.
+	 * Otherwise, disable or skip extra channels to match the new configuration.
+	 */
+	if (ses->chan_count < ses->chan_max)
+		rc = cifs_try_adding_channels(ses);
+	else
+		rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
+						from_reconfigure);
+
+	return rc;
+}
+
 static int
 smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 	       struct TCP_Server_Info *server, bool from_reconnect)
@@ -355,8 +382,8 @@ 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,
-					       from_reconnect);
+		rc = smb3_update_ses_channels(ses, server,
+					       from_reconnect, false /* from_reconfigure */);
 		if (rc) {
 			mutex_unlock(&ses->session_mutex);
 			goto out;
@@ -438,8 +465,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 			 * treat this as server not supporting multichannel
 			 */
 
-			rc = cifs_chan_skip_or_disable(ses, server,
-						       from_reconnect);
+			rc = smb3_update_ses_channels(ses, server,
+						       from_reconnect,
+						       false /* from_reconfigure */);
 			goto skip_add_channels;
 		} else if (rc)
 			cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
@@ -451,7 +479,8 @@ 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 /* from_reconfigure */);
 		}
 	} else {
 		mutex_unlock(&ses->session_mutex);
@@ -1105,8 +1134,7 @@ 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);
+	req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	/* ClientGUID must be zero for SMB2.02 dialect */
 	if (server->vals->protocol_id == SMB20_PROT_ID)
@@ -1312,8 +1340,7 @@ 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);
+	pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	memcpy(pneg_inbuf->Guid, server->client_guid,
 					SMB2_CLIENT_GUID_SIZE);
-- 
2.43.0

Re: [PATCH] cifs: client: allow changing multichannel mount options on remount

[Meetakshi Setiya]

Hi Rajasi,

On Tue, Nov 18, 2025 at 7:59 AM Rajasi Mandal <rajasimandalos@gmail.com> wrote:
>
> From: Rajasi Mandal <rajasimandal@microsoft.com>
>
> 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 from_reconfigure
>   flag to support targeted cleanup during reconfiguration.
> - 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>
> ---
>  fs/smb/client/cifsproto.h  |  4 ++-
>  fs/smb/client/connect.c    |  4 ++-
>  fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
>  fs/smb/client/sess.c       | 32 +++++++++++++++-------
>  fs/smb/client/smb2pdu.c    | 55 ++++++++++++++++++++++++++++----------
>  5 files changed, 119 insertions(+), 26 deletions(-)
>
> diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
> index 3528c365a452..a1fc9e1918bc 100644
> --- a/fs/smb/client/cifsproto.h
> +++ b/fs/smb/client/cifsproto.h
> @@ -649,6 +649,8 @@ 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 from_reconfigure);
>  bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
>  void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
>
> @@ -674,7 +676,7 @@ 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 from_reconfigure);
>  void
>  cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
>  int
> diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
> index 55cb4b0cbd48..cebe4a5f54f2 100644
> --- a/fs/smb/client/connect.c
> +++ b/fs/smb/client/connect.c
> @@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);

Shouldn't this be added to the non-dfs cifs_mount() too? I see that
even cifs_try_adding_channels() is not present in this function,
@Shyam Prasad is this expected?

>         rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
>         if (rc)
>                 goto error;
> diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
> index 0f894d09157b..751ed6ebd458 100644
> --- a/fs/smb/client/fs_context.c
> +++ b/fs/smb/client/fs_context.c
> @@ -717,6 +717,7 @@ 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 = {
> @@ -1013,6 +1014,22 @@ 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);
> @@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
>                 ses->password2 = new_password2;
>         }
>
> -       mutex_unlock(&ses->session_mutex);
> +       /*
> +        * 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);
> +                       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 */,
> +                                              true /* from_reconfigure */);
> +
> +               /* 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);
> diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
> index ef3b498b0a02..cfd83986a84a 100644
> --- a/fs/smb/client/sess.c
> +++ b/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.
> - * this always gets called from smb2_reconnect
> - * and cannot get called in parallel threads.
> + * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
> + * @ses: pointer to the CIFS session structure
> + * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
> + *
> + * 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 from_reconfigure)
>  {
>         int i, chan_count;
>         struct TCP_Server_Info *server;
> @@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
>         if (chan_count == 1)
>                 goto done;
>
> -       ses->chan_count = 1;
> -
> -       /* for all secondary channels reset the need reconnect bit */
> -       ses->chans_need_reconnect &= 1;
> +       /* Update the chan_count to the new maximum */
> +       if (from_reconfigure)
> +               ses->chan_count = ses->chan_max;
> +       else
> +               ses->chan_count = 1;
>
> -       for (i = 1; i < chan_count; i++) {
> +       for (i = ses->chan_max ; i < chan_count; i++) {
>                 iface = ses->chans[i].iface;
>                 server = ses->chans[i].server;
>
> @@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
>  }
> diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
> index 8b4a4573e9c3..d051da46eaab 100644
> --- a/fs/smb/client/smb2pdu.c
> +++ b/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 from_reconfigure)
>  {
>         struct TCP_Server_Info *pserver;
>         unsigned int chan_index;
> @@ -206,14 +206,41 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
>                 return -EHOSTDOWN;
>         }
>
> -       cifs_server_dbg(VFS,
> -               "server does not support multichannel anymore. Disable all other channels\n");
> -       cifs_disable_secondary_channels(ses);
> -
> +       cifs_decrease_secondary_channels(ses, from_reconfigure);
>
>         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
> + * @from_reconfigure: indicates if called from reconfigure context
> + *
> + * Returns 0 on success or error code on failure.
> + *
> + * Note: Outside of reconfigure, this function is only called from reconnect scenarios
> + * when the server stops advertising multichannel (MC) capability.
> + */

I see that function is being called during mount too. Did you
mean cifs_decrease_secondary_channels()?

> +int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
> +                       bool from_reconnect, bool from_reconfigure)
> +{
> +       int rc = 0;
> +       /*
> +        * If the current channel count is less than the new chan_max,
> +        * try to add channels to reach the new maximum.
> +        * Otherwise, disable or skip extra channels to match the new configuration.
> +        */
> +       if (ses->chan_count < ses->chan_max)
> +               rc = cifs_try_adding_channels(ses);
> +       else
> +               rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
> +                                               from_reconfigure);
> +
> +       return rc;
> +}
> +
>  static int
>  smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
>                struct TCP_Server_Info *server, bool from_reconnect)
> @@ -355,8 +382,8 @@ 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,
> -                                              from_reconnect);
> +               rc = smb3_update_ses_channels(ses, server,
> +                                              from_reconnect, false /* from_reconfigure */);
>                 if (rc) {
>                         mutex_unlock(&ses->session_mutex);
>                         goto out;
> @@ -438,8 +465,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
>                          * treat this as server not supporting multichannel
>                          */
>
> -                       rc = cifs_chan_skip_or_disable(ses, server,
> -                                                      from_reconnect);
> +                       rc = smb3_update_ses_channels(ses, server,
> +                                                      from_reconnect,
> +                                                      false /* from_reconfigure */);
>                         goto skip_add_channels;
>                 } else if (rc)
>                         cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
> @@ -451,7 +479,8 @@ 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 /* from_reconfigure */);
>                 }
>         } else {
>                 mutex_unlock(&ses->session_mutex);
> @@ -1105,8 +1134,7 @@ 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);
> +       req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
>
>         /* ClientGUID must be zero for SMB2.02 dialect */
>         if (server->vals->protocol_id == SMB20_PROT_ID)
> @@ -1312,8 +1340,7 @@ 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);
> +       pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
>
>         memcpy(pneg_inbuf->Guid, server->client_guid,
>                                         SMB2_CLIENT_GUID_SIZE);
> --
> 2.43.0
>
>

Thanks
Meetakshi

Re: [PATCH] cifs: client: allow changing multichannel mount options on remount

[kernel test robot]

BCC: lkp@intel.com
CC: oe-kbuild-all@lists.linux.dev
In-Reply-To: <20251118022655.126994-1-rajasimandalos@gmail.com>
References: <20251118022655.126994-1-rajasimandalos@gmail.com>
TO: Rajasi Mandal <rajasimandalos@gmail.com>

Hi Rajasi,

kernel test robot noticed the following build warnings:

[auto build test WARNING on cifs/for-next]
[also build test WARNING on linus/master v6.18-rc6 next-20251121]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Rajasi-Mandal/cifs-client-allow-changing-multichannel-mount-options-on-remount/20251118-102841
base:   git://git.samba.org/sfrench/cifs-2.6.git for-next
patch link:    https://lore.kernel.org/r/20251118022655.126994-1-rajasimandalos%40gmail.com
patch subject: [PATCH] cifs: client: allow changing multichannel mount options on remount
:::::: branch date: 4 days ago
:::::: commit date: 4 days ago
config: i386-randconfig-141-20251119 (https://download.01.org/0day-ci/archive/20251122/202511220407.4PHDdCbQ-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Reported-by: Dan Carpenter <error27@gmail.com>
| Closes: https://lore.kernel.org/r/202511220407.4PHDdCbQ-lkp@intel.com/

smatch warnings:
fs/smb/client/fs_context.c:1164 smb3_reconfigure() warn: inconsistent returns '&ses->session_mutex'.

vim +1164 fs/smb/client/fs_context.c

d6a78783407cdf fs/cifs/fs_context.c Ronnie Sahlberg 2020-12-14  1163  
d6a78783407cdf fs/cifs/fs_context.c Ronnie Sahlberg 2020-12-14 @1164  	return rc;
d6a78783407cdf fs/cifs/fs_context.c Ronnie Sahlberg 2020-12-14  1165  }
d6a78783407cdf fs/cifs/fs_context.c Ronnie Sahlberg 2020-12-14  1166  

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] cifs: client: allow changing multichannel mount options on remount

[Shyam Prasad N]

On Thu, Nov 20, 2025 at 11:40 AM Meetakshi Setiya
<meetakshisetiyaoss@gmail.com> wrote:
>
> Hi Rajasi,
>
> On Tue, Nov 18, 2025 at 7:59 AM Rajasi Mandal <rajasimandalos@gmail.com> wrote:
> >
> > From: Rajasi Mandal <rajasimandal@microsoft.com>
> >
> > 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 from_reconfigure
> >   flag to support targeted cleanup during reconfiguration.
> > - 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>
> > ---
> >  fs/smb/client/cifsproto.h  |  4 ++-
> >  fs/smb/client/connect.c    |  4 ++-
> >  fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
> >  fs/smb/client/sess.c       | 32 +++++++++++++++-------
> >  fs/smb/client/smb2pdu.c    | 55 ++++++++++++++++++++++++++++----------
> >  5 files changed, 119 insertions(+), 26 deletions(-)
> >
> > diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
> > index 3528c365a452..a1fc9e1918bc 100644
> > --- a/fs/smb/client/cifsproto.h
> > +++ b/fs/smb/client/cifsproto.h
> > @@ -649,6 +649,8 @@ 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 from_reconfigure);
> >  bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
> >  void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
> >
> > @@ -674,7 +676,7 @@ 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 from_reconfigure);
> >  void
> >  cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
> >  int
> > diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
> > index 55cb4b0cbd48..cebe4a5f54f2 100644
> > --- a/fs/smb/client/connect.c
> > +++ b/fs/smb/client/connect.c
> > @@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
>
> Shouldn't this be added to the non-dfs cifs_mount() too? I see that
> even cifs_try_adding_channels() is not present in this function,
> @Shyam Prasad is this expected?

That's a good catch.
I think this is a day-0 bug.
I can see that the original change by Aurelien has the change only in
DFS cifs_mount:
commit d70e9fa55884760b6d6c293dbf20d8c52ce11fb7
Author: Aurelien Aptel <aaptel@suse.com>
Date:   Fri Sep 20 06:31:10 2019 +0200

    cifs: try opening channels after mounting

We should have a follow-up patch to fix this.

>
> >         rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
> >         if (rc)
> >                 goto error;
> > diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
> > index 0f894d09157b..751ed6ebd458 100644
> > --- a/fs/smb/client/fs_context.c
> > +++ b/fs/smb/client/fs_context.c
> > @@ -717,6 +717,7 @@ 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 = {
> > @@ -1013,6 +1014,22 @@ 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);
> > @@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
> >                 ses->password2 = new_password2;
> >         }
> >
> > -       mutex_unlock(&ses->session_mutex);
> > +       /*
> > +        * 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);
> > +                       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 */,
> > +                                              true /* from_reconfigure */);
> > +
> > +               /* 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);
> > diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
> > index ef3b498b0a02..cfd83986a84a 100644
> > --- a/fs/smb/client/sess.c
> > +++ b/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.
> > - * this always gets called from smb2_reconnect
> > - * and cannot get called in parallel threads.
> > + * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
> > + * @ses: pointer to the CIFS session structure
> > + * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
> > + *
> > + * 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 from_reconfigure)
> >  {
> >         int i, chan_count;
> >         struct TCP_Server_Info *server;
> > @@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
> >         if (chan_count == 1)
> >                 goto done;
> >
> > -       ses->chan_count = 1;
> > -
> > -       /* for all secondary channels reset the need reconnect bit */
> > -       ses->chans_need_reconnect &= 1;
> > +       /* Update the chan_count to the new maximum */
> > +       if (from_reconfigure)
> > +               ses->chan_count = ses->chan_max;
> > +       else
> > +               ses->chan_count = 1;
> >
> > -       for (i = 1; i < chan_count; i++) {
> > +       for (i = ses->chan_max ; i < chan_count; i++) {
> >                 iface = ses->chans[i].iface;
> >                 server = ses->chans[i].server;
> >
> > @@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
> >  }
> > diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
> > index 8b4a4573e9c3..d051da46eaab 100644
> > --- a/fs/smb/client/smb2pdu.c
> > +++ b/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 from_reconfigure)
> >  {
> >         struct TCP_Server_Info *pserver;
> >         unsigned int chan_index;
> > @@ -206,14 +206,41 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
> >                 return -EHOSTDOWN;
> >         }
> >
> > -       cifs_server_dbg(VFS,
> > -               "server does not support multichannel anymore. Disable all other channels\n");
> > -       cifs_disable_secondary_channels(ses);
> > -
> > +       cifs_decrease_secondary_channels(ses, from_reconfigure);
> >
> >         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
> > + * @from_reconfigure: indicates if called from reconfigure context
> > + *
> > + * Returns 0 on success or error code on failure.
> > + *
> > + * Note: Outside of reconfigure, this function is only called from reconnect scenarios
> > + * when the server stops advertising multichannel (MC) capability.
> > + */
>
> I see that function is being called during mount too. Did you
> mean cifs_decrease_secondary_channels()?
>
> > +int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
> > +                       bool from_reconnect, bool from_reconfigure)
> > +{
> > +       int rc = 0;
> > +       /*
> > +        * If the current channel count is less than the new chan_max,
> > +        * try to add channels to reach the new maximum.
> > +        * Otherwise, disable or skip extra channels to match the new configuration.
> > +        */
> > +       if (ses->chan_count < ses->chan_max)
> > +               rc = cifs_try_adding_channels(ses);
> > +       else
> > +               rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
> > +                                               from_reconfigure);
> > +
> > +       return rc;
> > +}
> > +
> >  static int
> >  smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
> >                struct TCP_Server_Info *server, bool from_reconnect)
> > @@ -355,8 +382,8 @@ 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,
> > -                                              from_reconnect);
> > +               rc = smb3_update_ses_channels(ses, server,
> > +                                              from_reconnect, false /* from_reconfigure */);
> >                 if (rc) {
> >                         mutex_unlock(&ses->session_mutex);
> >                         goto out;
> > @@ -438,8 +465,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
> >                          * treat this as server not supporting multichannel
> >                          */
> >
> > -                       rc = cifs_chan_skip_or_disable(ses, server,
> > -                                                      from_reconnect);
> > +                       rc = smb3_update_ses_channels(ses, server,
> > +                                                      from_reconnect,
> > +                                                      false /* from_reconfigure */);
> >                         goto skip_add_channels;
> >                 } else if (rc)
> >                         cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
> > @@ -451,7 +479,8 @@ 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 /* from_reconfigure */);
> >                 }
> >         } else {
> >                 mutex_unlock(&ses->session_mutex);
> > @@ -1105,8 +1134,7 @@ 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);
> > +       req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
> >
> >         /* ClientGUID must be zero for SMB2.02 dialect */
> >         if (server->vals->protocol_id == SMB20_PROT_ID)
> > @@ -1312,8 +1340,7 @@ 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);
> > +       pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
> >
> >         memcpy(pneg_inbuf->Guid, server->client_guid,
> >                                         SMB2_CLIENT_GUID_SIZE);
> > --
> > 2.43.0
> >
> >
>
> Thanks
> Meetakshi
>


-- 
Regards,
Shyam

[PATCH] cifs: client: allow changing multichannel mount options on remount

[rajasimandalos]

From: Rajasi Mandal <rajasimandal@microsoft.com>

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 from_reconfigure
  flag to support targeted cleanup during reconfiguration.
- 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>
---
 fs/smb/client/cifsproto.h  |  4 ++-
 fs/smb/client/connect.c    |  4 ++-
 fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
 fs/smb/client/sess.c       | 32 ++++++++++++++++------
 fs/smb/client/smb2pdu.c    | 56 ++++++++++++++++++++++++++++----------
 5 files changed, 120 insertions(+), 26 deletions(-)

diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
index 3528c365a452..a1fc9e1918bc 100644
--- a/fs/smb/client/cifsproto.h
+++ b/fs/smb/client/cifsproto.h
@@ -649,6 +649,8 @@ 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 from_reconfigure);
 bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
 void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
 
@@ -674,7 +676,7 @@ 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 from_reconfigure);
 void
 cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
 int
diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index 2f94d93b95e9..6557b3546398 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
 	rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
 	if (rc)
 		goto error;
diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 2a0d8b87bd8e..46428181699e 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -717,6 +717,7 @@ 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 = {
@@ -1013,6 +1014,22 @@ 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);
@@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
 		ses->password2 = new_password2;
 	}
 
-	mutex_unlock(&ses->session_mutex);
+	/*
+	 * 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);
+			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 */,
+					       true /* from_reconfigure */);
+
+		/* 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);
diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index ef3b498b0a02..cfd83986a84a 100644
--- a/fs/smb/client/sess.c
+++ b/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.
- * this always gets called from smb2_reconnect
- * and cannot get called in parallel threads.
+ * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
+ * @ses: pointer to the CIFS session structure
+ * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
+ *
+ * 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 from_reconfigure)
 {
 	int i, chan_count;
 	struct TCP_Server_Info *server;
@@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
 	if (chan_count == 1)
 		goto done;
 
-	ses->chan_count = 1;
-
-	/* for all secondary channels reset the need reconnect bit */
-	ses->chans_need_reconnect &= 1;
+	/* Update the chan_count to the new maximum */
+	if (from_reconfigure)
+		ses->chan_count = ses->chan_max;
+	else
+		ses->chan_count = 1;
 
-	for (i = 1; i < chan_count; i++) {
+	for (i = ses->chan_max ; i < chan_count; i++) {
 		iface = ses->chans[i].iface;
 		server = ses->chans[i].server;
 
@@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
 }
diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 8b4a4573e9c3..37a3216b37e4 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/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 from_reconfigure)
 {
 	struct TCP_Server_Info *pserver;
 	unsigned int chan_index;
@@ -206,14 +206,42 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
 		return -EHOSTDOWN;
 	}
 
-	cifs_server_dbg(VFS,
-		"server does not support multichannel anymore. Disable all other channels\n");
-	cifs_disable_secondary_channels(ses);
-
+	cifs_decrease_secondary_channels(ses, from_reconfigure);
 
 	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
+ * @from_reconfigure: indicates if called from reconfigure context
+ *
+ * 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 from_reconfigure)
+{
+	int rc = 0;
+	/*
+	 * If the current channel count is less than the new chan_max,
+	 * try to add channels to reach the new maximum.
+	 * Otherwise, disable or skip extra channels to match the new configuration.
+	 */
+	if (ses->chan_count < ses->chan_max)
+		rc = cifs_try_adding_channels(ses);
+	else
+		rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
+						from_reconfigure);
+
+	return rc;
+}
+
 static int
 smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 	       struct TCP_Server_Info *server, bool from_reconnect)
@@ -355,8 +383,8 @@ 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,
-					       from_reconnect);
+		rc = smb3_update_ses_channels(ses, server,
+					       from_reconnect, false /* from_reconfigure */);
 		if (rc) {
 			mutex_unlock(&ses->session_mutex);
 			goto out;
@@ -438,8 +466,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 			 * treat this as server not supporting multichannel
 			 */
 
-			rc = cifs_chan_skip_or_disable(ses, server,
-						       from_reconnect);
+			rc = smb3_update_ses_channels(ses, server,
+						       from_reconnect,
+						       false /* from_reconfigure */);
 			goto skip_add_channels;
 		} else if (rc)
 			cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
@@ -451,7 +480,8 @@ 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 /* from_reconfigure */);
 		}
 	} else {
 		mutex_unlock(&ses->session_mutex);
@@ -1105,8 +1135,7 @@ 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);
+	req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	/* ClientGUID must be zero for SMB2.02 dialect */
 	if (server->vals->protocol_id == SMB20_PROT_ID)
@@ -1312,8 +1341,7 @@ 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);
+	pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	memcpy(pneg_inbuf->Guid, server->client_guid,
 					SMB2_CLIENT_GUID_SIZE);
-- 
2.43.0

Re: [PATCH] cifs: client: allow changing multichannel mount options on remount

[RAJASI MANDAL]

[-- Attachment #1.1: Type: text/plain, Size: 17904 bytes --]

Sending a follow-up patch with corrected comment as pointed out by
Meetakshi. Attaching the new patch here for easy reference.

Regards,
Rajasi

On Mon, Nov 24, 2025 at 5:20 PM Shyam Prasad N <nspmangalore@gmail.com>
wrote:

> On Thu, Nov 20, 2025 at 11:40 AM Meetakshi Setiya
> <meetakshisetiyaoss@gmail.com> wrote:
> >
> > Hi Rajasi,
> >
> > On Tue, Nov 18, 2025 at 7:59 AM Rajasi Mandal <rajasimandalos@gmail.com>
> wrote:
> > >
> > > From: Rajasi Mandal <rajasimandal@microsoft.com>
> > >
> > > 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 from_reconfigure
> > >   flag to support targeted cleanup during reconfiguration.
> > > - 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>
> > > ---
> > >  fs/smb/client/cifsproto.h  |  4 ++-
> > >  fs/smb/client/connect.c    |  4 ++-
> > >  fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
> > >  fs/smb/client/sess.c       | 32 +++++++++++++++-------
> > >  fs/smb/client/smb2pdu.c    | 55 ++++++++++++++++++++++++++++----------
> > >  5 files changed, 119 insertions(+), 26 deletions(-)
> > >
> > > diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
> > > index 3528c365a452..a1fc9e1918bc 100644
> > > --- a/fs/smb/client/cifsproto.h
> > > +++ b/fs/smb/client/cifsproto.h
> > > @@ -649,6 +649,8 @@ 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
> from_reconfigure);
> > >  bool is_ses_using_iface(struct cifs_ses *ses, struct
> cifs_server_iface *iface);
> > >  void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
> > >
> > > @@ -674,7 +676,7 @@ 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
> from_reconfigure);
> > >  void
> > >  cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info
> *server);
> > >  int
> > > diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
> > > index 55cb4b0cbd48..cebe4a5f54f2 100644
> > > --- a/fs/smb/client/connect.c
> > > +++ b/fs/smb/client/connect.c
> > > @@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
> >
> > Shouldn't this be added to the non-dfs cifs_mount() too? I see that
> > even cifs_try_adding_channels() is not present in this function,
> > @Shyam Prasad is this expected?
>
> That's a good catch.
> I think this is a day-0 bug.
> I can see that the original change by Aurelien has the change only in
> DFS cifs_mount:
> commit d70e9fa55884760b6d6c293dbf20d8c52ce11fb7
> Author: Aurelien Aptel <aaptel@suse.com>
> Date:   Fri Sep 20 06:31:10 2019 +0200
>
>     cifs: try opening channels after mounting
>
> We should have a follow-up patch to fix this.
>
> >
> > >         rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
> > >         if (rc)
> > >                 goto error;
> > > diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
> > > index 0f894d09157b..751ed6ebd458 100644
> > > --- a/fs/smb/client/fs_context.c
> > > +++ b/fs/smb/client/fs_context.c
> > > @@ -717,6 +717,7 @@ 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 = {
> > > @@ -1013,6 +1014,22 @@ 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);
> > > @@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context
> *fc)
> > >                 ses->password2 = new_password2;
> > >         }
> > >
> > > -       mutex_unlock(&ses->session_mutex);
> > > +       /*
> > > +        * 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);
> > > +                       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
> */,
> > > +                                              true /*
> from_reconfigure */);
> > > +
> > > +               /* 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);
> > > diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
> > > index ef3b498b0a02..cfd83986a84a 100644
> > > --- a/fs/smb/client/sess.c
> > > +++ b/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.
> > > - * this always gets called from smb2_reconnect
> > > - * and cannot get called in parallel threads.
> > > + * cifs_decrease_secondary_channels - Reduce the number of active
> secondary channels
> > > + * @ses: pointer to the CIFS session structure
> > > + * @from_reconfigure: if true, only reduce to chan_max; if false,
> reduce to a single channel
> > > + *
> > > + * 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
> from_reconfigure)
> > >  {
> > >         int i, chan_count;
> > >         struct TCP_Server_Info *server;
> > > @@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses
> *ses)
> > >         if (chan_count == 1)
> > >                 goto done;
> > >
> > > -       ses->chan_count = 1;
> > > -
> > > -       /* for all secondary channels reset the need reconnect bit */
> > > -       ses->chans_need_reconnect &= 1;
> > > +       /* Update the chan_count to the new maximum */
> > > +       if (from_reconfigure)
> > > +               ses->chan_count = ses->chan_max;
> > > +       else
> > > +               ses->chan_count = 1;
> > >
> > > -       for (i = 1; i < chan_count; i++) {
> > > +       for (i = ses->chan_max ; i < chan_count; i++) {
> > >                 iface = ses->chans[i].iface;
> > >                 server = ses->chans[i].server;
> > >
> > > @@ -318,6 +323,15 @@ 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, "server does not support multichannel
> anymore. Disable all other 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);
> > >  }
> > > diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
> > > index 8b4a4573e9c3..d051da46eaab 100644
> > > --- a/fs/smb/client/smb2pdu.c
> > > +++ b/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 from_reconfigure)
> > >  {
> > >         struct TCP_Server_Info *pserver;
> > >         unsigned int chan_index;
> > > @@ -206,14 +206,41 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
> > >                 return -EHOSTDOWN;
> > >         }
> > >
> > > -       cifs_server_dbg(VFS,
> > > -               "server does not support multichannel anymore. Disable
> all other channels\n");
> > > -       cifs_disable_secondary_channels(ses);
> > > -
> > > +       cifs_decrease_secondary_channels(ses, from_reconfigure);
> > >
> > >         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
> > > + * @from_reconfigure: indicates if called from reconfigure context
> > > + *
> > > + * Returns 0 on success or error code on failure.
> > > + *
> > > + * Note: Outside of reconfigure, this function is only called from
> reconnect scenarios
> > > + * when the server stops advertising multichannel (MC) capability.
> > > + */
> >
> > I see that function is being called during mount too. Did you
> > mean cifs_decrease_secondary_channels()?
> >
> > > +int smb3_update_ses_channels(struct cifs_ses *ses, struct
> TCP_Server_Info *server,
> > > +                       bool from_reconnect, bool from_reconfigure)
> > > +{
> > > +       int rc = 0;
> > > +       /*
> > > +        * If the current channel count is less than the new chan_max,
> > > +        * try to add channels to reach the new maximum.
> > > +        * Otherwise, disable or skip extra channels to match the new
> configuration.
> > > +        */
> > > +       if (ses->chan_count < ses->chan_max)
> > > +               rc = cifs_try_adding_channels(ses);
> > > +       else
> > > +               rc = cifs_chan_skip_or_disable(ses, server,
> from_reconnect,
> > > +                                               from_reconfigure);
> > > +
> > > +       return rc;
> > > +}
> > > +
> > >  static int
> > >  smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
> > >                struct TCP_Server_Info *server, bool from_reconnect)
> > > @@ -355,8 +382,8 @@ 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,
> > > -                                              from_reconnect);
> > > +               rc = smb3_update_ses_channels(ses, server,
> > > +                                              from_reconnect, false
> /* from_reconfigure */);
> > >                 if (rc) {
> > >                         mutex_unlock(&ses->session_mutex);
> > >                         goto out;
> > > @@ -438,8 +465,9 @@ smb2_reconnect(__le16 smb2_command, struct
> cifs_tcon *tcon,
> > >                          * treat this as server not supporting
> multichannel
> > >                          */
> > >
> > > -                       rc = cifs_chan_skip_or_disable(ses, server,
> > > -                                                      from_reconnect);
> > > +                       rc = smb3_update_ses_channels(ses, server,
> > > +                                                      from_reconnect,
> > > +                                                      false /*
> from_reconfigure */);
> > >                         goto skip_add_channels;
> > >                 } else if (rc)
> > >                         cifs_tcon_dbg(FYI, "%s: failed to query server
> interfaces: %d\n",
> > > @@ -451,7 +479,8 @@ 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 /*
> from_reconfigure */);
> > >                 }
> > >         } else {
> > >                 mutex_unlock(&ses->session_mutex);
> > > @@ -1105,8 +1134,7 @@ 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);
> > > +       req->Capabilities |=
> cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
> > >
> > >         /* ClientGUID must be zero for SMB2.02 dialect */
> > >         if (server->vals->protocol_id == SMB20_PROT_ID)
> > > @@ -1312,8 +1340,7 @@ 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);
> > > +       pneg_inbuf->Capabilities |=
> cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
> > >
> > >         memcpy(pneg_inbuf->Guid, server->client_guid,
> > >                                         SMB2_CLIENT_GUID_SIZE);
> > > --
> > > 2.43.0
> > >
> > >
> >
> > Thanks
> > Meetakshi
> >
>
>
> --
> Regards,
> Shyam
>

[-- Attachment #1.2: Type: text/html, Size: 22897 bytes --]

[-- Attachment #2: [PATCH] cifs_ client_ allow changing multichannel mount options on remount (1).eml --]
[-- Type: message/rfc822, Size: 13344 bytes --]

From: rajasimandalos@gmail.com
To: linux-cifs@vger.kernel.org
Cc: sfrench@samba.org, pc@manguebit.org, ronniesahlberg@gmail.com, sprasad@microsoft.com, tom@talpey.com, bharathsm@microsoft.com, samba-technical@lists.samba.org, Rajasi Mandal <rajasimandal@microsoft.com>
Subject: [PATCH] cifs: client: allow changing multichannel mount options on remount
Date: Mon,  1 Dec 2025 06:25:17 +0000
Message-ID: <20251201062517.1513683-1-rajasimandalos@gmail.com>

From: Rajasi Mandal <rajasimandal@microsoft.com>

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 from_reconfigure
  flag to support targeted cleanup during reconfiguration.
- 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>
---
 fs/smb/client/cifsproto.h  |  4 ++-
 fs/smb/client/connect.c    |  4 ++-
 fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
 fs/smb/client/sess.c       | 32 ++++++++++++++++------
 fs/smb/client/smb2pdu.c    | 56 ++++++++++++++++++++++++++++----------
 5 files changed, 120 insertions(+), 26 deletions(-)

diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
index 3528c365a452..a1fc9e1918bc 100644
--- a/fs/smb/client/cifsproto.h
+++ b/fs/smb/client/cifsproto.h
@@ -649,6 +649,8 @@ 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 from_reconfigure);
 bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
 void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
 
@@ -674,7 +676,7 @@ 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 from_reconfigure);
 void
 cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
 int
diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index 2f94d93b95e9..6557b3546398 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
 	rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
 	if (rc)
 		goto error;
diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index 2a0d8b87bd8e..46428181699e 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -717,6 +717,7 @@ 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 = {
@@ -1013,6 +1014,22 @@ 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);
@@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
 		ses->password2 = new_password2;
 	}
 
-	mutex_unlock(&ses->session_mutex);
+	/*
+	 * 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);
+			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 */,
+					       true /* from_reconfigure */);
+
+		/* 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);
diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
index ef3b498b0a02..cfd83986a84a 100644
--- a/fs/smb/client/sess.c
+++ b/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.
- * this always gets called from smb2_reconnect
- * and cannot get called in parallel threads.
+ * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
+ * @ses: pointer to the CIFS session structure
+ * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
+ *
+ * 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 from_reconfigure)
 {
 	int i, chan_count;
 	struct TCP_Server_Info *server;
@@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
 	if (chan_count == 1)
 		goto done;
 
-	ses->chan_count = 1;
-
-	/* for all secondary channels reset the need reconnect bit */
-	ses->chans_need_reconnect &= 1;
+	/* Update the chan_count to the new maximum */
+	if (from_reconfigure)
+		ses->chan_count = ses->chan_max;
+	else
+		ses->chan_count = 1;
 
-	for (i = 1; i < chan_count; i++) {
+	for (i = ses->chan_max ; i < chan_count; i++) {
 		iface = ses->chans[i].iface;
 		server = ses->chans[i].server;
 
@@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
 }
diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
index 8b4a4573e9c3..37a3216b37e4 100644
--- a/fs/smb/client/smb2pdu.c
+++ b/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 from_reconfigure)
 {
 	struct TCP_Server_Info *pserver;
 	unsigned int chan_index;
@@ -206,14 +206,42 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
 		return -EHOSTDOWN;
 	}
 
-	cifs_server_dbg(VFS,
-		"server does not support multichannel anymore. Disable all other channels\n");
-	cifs_disable_secondary_channels(ses);
-
+	cifs_decrease_secondary_channels(ses, from_reconfigure);
 
 	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
+ * @from_reconfigure: indicates if called from reconfigure context
+ *
+ * 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 from_reconfigure)
+{
+	int rc = 0;
+	/*
+	 * If the current channel count is less than the new chan_max,
+	 * try to add channels to reach the new maximum.
+	 * Otherwise, disable or skip extra channels to match the new configuration.
+	 */
+	if (ses->chan_count < ses->chan_max)
+		rc = cifs_try_adding_channels(ses);
+	else
+		rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
+						from_reconfigure);
+
+	return rc;
+}
+
 static int
 smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 	       struct TCP_Server_Info *server, bool from_reconnect)
@@ -355,8 +383,8 @@ 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,
-					       from_reconnect);
+		rc = smb3_update_ses_channels(ses, server,
+					       from_reconnect, false /* from_reconfigure */);
 		if (rc) {
 			mutex_unlock(&ses->session_mutex);
 			goto out;
@@ -438,8 +466,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
 			 * treat this as server not supporting multichannel
 			 */
 
-			rc = cifs_chan_skip_or_disable(ses, server,
-						       from_reconnect);
+			rc = smb3_update_ses_channels(ses, server,
+						       from_reconnect,
+						       false /* from_reconfigure */);
 			goto skip_add_channels;
 		} else if (rc)
 			cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
@@ -451,7 +480,8 @@ 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 /* from_reconfigure */);
 		}
 	} else {
 		mutex_unlock(&ses->session_mutex);
@@ -1105,8 +1135,7 @@ 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);
+	req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	/* ClientGUID must be zero for SMB2.02 dialect */
 	if (server->vals->protocol_id == SMB20_PROT_ID)
@@ -1312,8 +1341,7 @@ 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);
+	pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
 
 	memcpy(pneg_inbuf->Guid, server->client_guid,
 					SMB2_CLIENT_GUID_SIZE);
-- 
2.43.0

Re: [PATCH] cifs: client: allow changing multichannel mount options on remount

[Steve French]

merged the two patches into ksmbd-for-next (will move to cifs-2.6.git
for-next once the next P/R is complete, but want to avoid merge
conflicts for now) pending additional testing and review

On Mon, Dec 1, 2025 at 3:21 AM RAJASI MANDAL <rajasimandalos@gmail.com> wrote:
>
> Sending a follow-up patch with corrected comment as pointed out by Meetakshi. Attaching the new patch here for easy reference.
>
> Regards,
> Rajasi
>
> On Mon, Nov 24, 2025 at 5:20 PM Shyam Prasad N <nspmangalore@gmail.com> wrote:
>>
>> On Thu, Nov 20, 2025 at 11:40 AM Meetakshi Setiya
>> <meetakshisetiyaoss@gmail.com> wrote:
>> >
>> > Hi Rajasi,
>> >
>> > On Tue, Nov 18, 2025 at 7:59 AM Rajasi Mandal <rajasimandalos@gmail.com> wrote:
>> > >
>> > > From: Rajasi Mandal <rajasimandal@microsoft.com>
>> > >
>> > > 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 from_reconfigure
>> > >   flag to support targeted cleanup during reconfiguration.
>> > > - 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>
>> > > ---
>> > >  fs/smb/client/cifsproto.h  |  4 ++-
>> > >  fs/smb/client/connect.c    |  4 ++-
>> > >  fs/smb/client/fs_context.c | 50 +++++++++++++++++++++++++++++++++-
>> > >  fs/smb/client/sess.c       | 32 +++++++++++++++-------
>> > >  fs/smb/client/smb2pdu.c    | 55 ++++++++++++++++++++++++++++----------
>> > >  5 files changed, 119 insertions(+), 26 deletions(-)
>> > >
>> > > diff --git a/fs/smb/client/cifsproto.h b/fs/smb/client/cifsproto.h
>> > > index 3528c365a452..a1fc9e1918bc 100644
>> > > --- a/fs/smb/client/cifsproto.h
>> > > +++ b/fs/smb/client/cifsproto.h
>> > > @@ -649,6 +649,8 @@ 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 from_reconfigure);
>> > >  bool is_ses_using_iface(struct cifs_ses *ses, struct cifs_server_iface *iface);
>> > >  void cifs_ses_mark_for_reconnect(struct cifs_ses *ses);
>> > >
>> > > @@ -674,7 +676,7 @@ 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 from_reconfigure);
>> > >  void
>> > >  cifs_chan_update_iface(struct cifs_ses *ses, struct TCP_Server_Info *server);
>> > >  int
>> > > diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
>> > > index 55cb4b0cbd48..cebe4a5f54f2 100644
>> > > --- a/fs/smb/client/connect.c
>> > > +++ b/fs/smb/client/connect.c
>> > > @@ -3927,7 +3927,9 @@ 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 /* from_reconfigure */);
>> >
>> > Shouldn't this be added to the non-dfs cifs_mount() too? I see that
>> > even cifs_try_adding_channels() is not present in this function,
>> > @Shyam Prasad is this expected?
>>
>> That's a good catch.
>> I think this is a day-0 bug.
>> I can see that the original change by Aurelien has the change only in
>> DFS cifs_mount:
>> commit d70e9fa55884760b6d6c293dbf20d8c52ce11fb7
>> Author: Aurelien Aptel <aaptel@suse.com>
>> Date:   Fri Sep 20 06:31:10 2019 +0200
>>
>>     cifs: try opening channels after mounting
>>
>> We should have a follow-up patch to fix this.
>>
>> >
>> > >         rc = mount_setup_tlink(cifs_sb, mnt_ctx.ses, mnt_ctx.tcon);
>> > >         if (rc)
>> > >                 goto error;
>> > > diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
>> > > index 0f894d09157b..751ed6ebd458 100644
>> > > --- a/fs/smb/client/fs_context.c
>> > > +++ b/fs/smb/client/fs_context.c
>> > > @@ -717,6 +717,7 @@ 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 = {
>> > > @@ -1013,6 +1014,22 @@ 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);
>> > > @@ -1095,7 +1112,38 @@ static int smb3_reconfigure(struct fs_context *fc)
>> > >                 ses->password2 = new_password2;
>> > >         }
>> > >
>> > > -       mutex_unlock(&ses->session_mutex);
>> > > +       /*
>> > > +        * 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);
>> > > +                       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 */,
>> > > +                                              true /* from_reconfigure */);
>> > > +
>> > > +               /* 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);
>> > > diff --git a/fs/smb/client/sess.c b/fs/smb/client/sess.c
>> > > index ef3b498b0a02..cfd83986a84a 100644
>> > > --- a/fs/smb/client/sess.c
>> > > +++ b/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.
>> > > - * this always gets called from smb2_reconnect
>> > > - * and cannot get called in parallel threads.
>> > > + * cifs_decrease_secondary_channels - Reduce the number of active secondary channels
>> > > + * @ses: pointer to the CIFS session structure
>> > > + * @from_reconfigure: if true, only reduce to chan_max; if false, reduce to a single channel
>> > > + *
>> > > + * 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 from_reconfigure)
>> > >  {
>> > >         int i, chan_count;
>> > >         struct TCP_Server_Info *server;
>> > > @@ -281,12 +285,13 @@ cifs_disable_secondary_channels(struct cifs_ses *ses)
>> > >         if (chan_count == 1)
>> > >                 goto done;
>> > >
>> > > -       ses->chan_count = 1;
>> > > -
>> > > -       /* for all secondary channels reset the need reconnect bit */
>> > > -       ses->chans_need_reconnect &= 1;
>> > > +       /* Update the chan_count to the new maximum */
>> > > +       if (from_reconfigure)
>> > > +               ses->chan_count = ses->chan_max;
>> > > +       else
>> > > +               ses->chan_count = 1;
>> > >
>> > > -       for (i = 1; i < chan_count; i++) {
>> > > +       for (i = ses->chan_max ; i < chan_count; i++) {
>> > >                 iface = ses->chans[i].iface;
>> > >                 server = ses->chans[i].server;
>> > >
>> > > @@ -318,6 +323,15 @@ 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, "server does not support multichannel anymore. Disable all other 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);
>> > >  }
>> > > diff --git a/fs/smb/client/smb2pdu.c b/fs/smb/client/smb2pdu.c
>> > > index 8b4a4573e9c3..d051da46eaab 100644
>> > > --- a/fs/smb/client/smb2pdu.c
>> > > +++ b/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 from_reconfigure)
>> > >  {
>> > >         struct TCP_Server_Info *pserver;
>> > >         unsigned int chan_index;
>> > > @@ -206,14 +206,41 @@ cifs_chan_skip_or_disable(struct cifs_ses *ses,
>> > >                 return -EHOSTDOWN;
>> > >         }
>> > >
>> > > -       cifs_server_dbg(VFS,
>> > > -               "server does not support multichannel anymore. Disable all other channels\n");
>> > > -       cifs_disable_secondary_channels(ses);
>> > > -
>> > > +       cifs_decrease_secondary_channels(ses, from_reconfigure);
>> > >
>> > >         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
>> > > + * @from_reconfigure: indicates if called from reconfigure context
>> > > + *
>> > > + * Returns 0 on success or error code on failure.
>> > > + *
>> > > + * Note: Outside of reconfigure, this function is only called from reconnect scenarios
>> > > + * when the server stops advertising multichannel (MC) capability.
>> > > + */
>> >
>> > I see that function is being called during mount too. Did you
>> > mean cifs_decrease_secondary_channels()?
>> >
>> > > +int smb3_update_ses_channels(struct cifs_ses *ses, struct TCP_Server_Info *server,
>> > > +                       bool from_reconnect, bool from_reconfigure)
>> > > +{
>> > > +       int rc = 0;
>> > > +       /*
>> > > +        * If the current channel count is less than the new chan_max,
>> > > +        * try to add channels to reach the new maximum.
>> > > +        * Otherwise, disable or skip extra channels to match the new configuration.
>> > > +        */
>> > > +       if (ses->chan_count < ses->chan_max)
>> > > +               rc = cifs_try_adding_channels(ses);
>> > > +       else
>> > > +               rc = cifs_chan_skip_or_disable(ses, server, from_reconnect,
>> > > +                                               from_reconfigure);
>> > > +
>> > > +       return rc;
>> > > +}
>> > > +
>> > >  static int
>> > >  smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
>> > >                struct TCP_Server_Info *server, bool from_reconnect)
>> > > @@ -355,8 +382,8 @@ 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,
>> > > -                                              from_reconnect);
>> > > +               rc = smb3_update_ses_channels(ses, server,
>> > > +                                              from_reconnect, false /* from_reconfigure */);
>> > >                 if (rc) {
>> > >                         mutex_unlock(&ses->session_mutex);
>> > >                         goto out;
>> > > @@ -438,8 +465,9 @@ smb2_reconnect(__le16 smb2_command, struct cifs_tcon *tcon,
>> > >                          * treat this as server not supporting multichannel
>> > >                          */
>> > >
>> > > -                       rc = cifs_chan_skip_or_disable(ses, server,
>> > > -                                                      from_reconnect);
>> > > +                       rc = smb3_update_ses_channels(ses, server,
>> > > +                                                      from_reconnect,
>> > > +                                                      false /* from_reconfigure */);
>> > >                         goto skip_add_channels;
>> > >                 } else if (rc)
>> > >                         cifs_tcon_dbg(FYI, "%s: failed to query server interfaces: %d\n",
>> > > @@ -451,7 +479,8 @@ 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 /* from_reconfigure */);
>> > >                 }
>> > >         } else {
>> > >                 mutex_unlock(&ses->session_mutex);
>> > > @@ -1105,8 +1134,7 @@ 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);
>> > > +       req->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
>> > >
>> > >         /* ClientGUID must be zero for SMB2.02 dialect */
>> > >         if (server->vals->protocol_id == SMB20_PROT_ID)
>> > > @@ -1312,8 +1340,7 @@ 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);
>> > > +       pneg_inbuf->Capabilities |= cpu_to_le32(SMB2_GLOBAL_CAP_MULTI_CHANNEL);
>> > >
>> > >         memcpy(pneg_inbuf->Guid, server->client_guid,
>> > >                                         SMB2_CLIENT_GUID_SIZE);
>> > > --
>> > > 2.43.0
>> > >
>> > >
>> >
>> > Thanks
>> > Meetakshi
>> >
>>
>>
>> --
>> Regards,
>> Shyam



-- 
Thanks,

Steve