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Message-ID: <CANT5p=ohMvQ+0+YYCQrkSw7sPW24gs60TQzBoiAyfedO8GXgfQ@mail.gmail.com>
Date: Mon, 24 Nov 2025 17:19:58 +0530
From: Shyam Prasad N <nspmangalore@...il.com>
To: Meetakshi Setiya <meetakshisetiyaoss@...il.com>
Cc: Rajasi Mandal <rajasimandalos@...il.com>, sfrench@...ba.org, linux-cifs@...r.kernel.org,
pc@...guebit.org, ronniesahlberg@...il.com, sprasad@...rosoft.com,
tom@...pey.com, bharathsm@...rosoft.com, samba-technical@...ts.samba.org,
linux-kernel@...r.kernel.org, Rajasi Mandal <rajasimandal@...rosoft.com>
Subject: Re: [PATCH] cifs: client: allow changing multichannel mount options
on remount
On Thu, Nov 20, 2025 at 11:40 AM Meetakshi Setiya
<meetakshisetiyaoss@...il.com> wrote:
>
> Hi Rajasi,
>
> On Tue, Nov 18, 2025 at 7:59 AM Rajasi Mandal <rajasimandalos@...il.com> wrote:
> >
> > From: Rajasi Mandal <rajasimandal@...rosoft.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@...rosoft.com>
> > Signed-off-by: Rajasi Mandal <rajasimandal@...rosoft.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@...e.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
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