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Message-ID: <7mc3cpg6qojvq7hak6jvkud7xgynmaki554tgn2jic2y52onzm@ugw7wsq43wsp>
Date: Tue, 18 Nov 2025 11:00:07 -0300
From: Enzo Matsumiya <ematsumiya@...e.de>
To: Rajasi Mandal <rajasimandalos@...il.com>
Cc: 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: enforce consistent handling of
multichannel and max_channels
On 11/18, Rajasi Mandal wrote:
>From: Rajasi Mandal <rajasimandal@...rosoft.com>
>
>Previously, the behavior of the multichannel and max_channels mount
>options was inconsistent and order-dependent. For example, specifying
>"multichannel,max_channels=1" would result in 2 channels, while
>"max_channels=1,multichannel" would result in 1 channel. Additionally,
>conflicting combinations such as "nomultichannel,max_channels=3" or
>"multichannel,max_channels=1" did not produce errors and could lead to
>unexpected channel counts.
>
>This commit introduces two new fields in smb3_fs_context to explicitly
>track whether multichannel and max_channels were specified during
>mount. The option parsing and validation logic is updated to ensure:
>- The outcome is no longer dependent on the order of options.
>- Conflicting combinations (e.g., "nomultichannel,max_channels=3" or
> "multichannel,max_channels=1") are detected and result in an error.
>- The number of channels created is consistent with the specified
> options.
>
>This improves the reliability and predictability of mount option
>handling for SMB3 multichannel support.
>
>Reviewed-by: Shyam Prasad N <sprasad@...rosoft.com>
>Signed-off-by: Rajasi Mandal <rajasimandal@...rosoft.com>
It's conflicting because it's already too complex for something that
should've been simple. This patch introduces a new field + unnecessary
logic on top if it all.
cf. a PoC patch I sent a while ago, we can (ab)use fsparam with same key
name, but different key types, so we could only deal with:
'nomultichannel', 'multichannel={0,1,off,no}' as multichannel disabled
'multichannel' as ctx->max_channels=2 (multichannel enabled, obviously)
'multichannel=X' as ctx->max_channels=X (ditto)
Makes 0 sense to have both multichannel and max_channels mount options.
>---
> fs/smb/client/cifsfs.c | 1 -
> fs/smb/client/fs_context.c | 65 ++++++++++++++++++++++++++++----------
> fs/smb/client/fs_context.h | 2 ++
> 3 files changed, 50 insertions(+), 18 deletions(-)
>
>diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
>index 185ac41bd7e9..4d53ec53d8db 100644
>--- a/fs/smb/client/cifsfs.c
>+++ b/fs/smb/client/cifsfs.c
>@@ -1016,7 +1016,6 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
> } else {
> cifs_info("Attempting to mount %s\n", old_ctx->source);
> }
>-
> cifs_sb = kzalloc(sizeof(*cifs_sb), GFP_KERNEL);
> if (!cifs_sb)
> return ERR_PTR(-ENOMEM);
>diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
>index 46d1eaae62da..1794a31541fe 100644
>--- a/fs/smb/client/fs_context.c
>+++ b/fs/smb/client/fs_context.c
>@@ -711,6 +711,47 @@ smb3_parse_devname(const char *devname, struct smb3_fs_context *ctx)
> return 0;
> }
>
>+static int smb3_handle_conflicting_options(struct fs_context *fc)
>+{
>+ struct smb3_fs_context *ctx = smb3_fc2context(fc);
>+
>+ if (ctx->multichannel_specified) {
>+ if (ctx->multichannel) {
>+ if (!ctx->max_channels_specified) {
>+ ctx->max_channels = 2;
>+ } else if (ctx->max_channels == 1) {
>+ cifs_errorf(fc,
>+ "max_channels must be greater than 1 when multichannel is enabled\n");
>+ return -EINVAL;
>+ }
>+ } else {
>+ if (!ctx->max_channels_specified) {
>+ ctx->max_channels = 1;
>+ } else if (ctx->max_channels > 1) {
>+ cifs_errorf(fc,
>+ "max_channels must be equal to 1 when multichannel is disabled\n");
>+ return -EINVAL;
>+ }
>+ }
>+ } else {
>+ if (ctx->max_channels_specified) {
>+ if (ctx->max_channels > 1)
>+ ctx->multichannel = true;
>+ else
>+ ctx->multichannel = false;
>+ } else {
>+ ctx->multichannel = false;
>+ ctx->max_channels = 1;
>+ }
>+ }
>+
>+ //resetting default values as remount doesn't initialize fs_context again
Please stick to /* ... */ comments style.
>+ ctx->multichannel_specified = false;
>+ ctx->max_channels_specified = false;
>+
>+ return 0;
>+}
>+
> static void smb3_fs_context_free(struct fs_context *fc);
> static int smb3_fs_context_parse_param(struct fs_context *fc,
> struct fs_parameter *param);
>@@ -785,6 +826,7 @@ static int smb3_fs_context_parse_monolithic(struct fs_context *fc,
> if (ret < 0)
> break;
> }
>+ ret = smb3_handle_conflicting_options(fc);
>
> return ret;
> }
>@@ -1296,15 +1338,11 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
> ctx->nodelete = 1;
> break;
> case Opt_multichannel:
>- if (result.negated) {
>+ ctx->multichannel_specified = true;
>+ if (result.negated)
> ctx->multichannel = false;
>- ctx->max_channels = 1;
>- } else {
>+ else
> ctx->multichannel = true;
>- /* if number of channels not specified, default to 2 */
>- if (ctx->max_channels < 2)
>- ctx->max_channels = 2;
>- }
> break;
> case Opt_uid:
> ctx->linux_uid = result.uid;
>@@ -1440,15 +1478,13 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
> ctx->max_credits = result.uint_32;
> break;
> case Opt_max_channels:
>+ ctx->max_channels_specified = true;
> if (result.uint_32 < 1 || result.uint_32 > CIFS_MAX_CHANNELS) {
> cifs_errorf(fc, "%s: Invalid max_channels value, needs to be 1-%d\n",
> __func__, CIFS_MAX_CHANNELS);
> goto cifs_parse_mount_err;
> }
> ctx->max_channels = result.uint_32;
>- /* If more than one channel requested ... they want multichan */
>- if (result.uint_32 > 1)
>- ctx->multichannel = true;
> break;
> case Opt_max_cached_dirs:
> if (result.uint_32 < 1) {
>@@ -1866,13 +1902,6 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
> goto cifs_parse_mount_err;
> }
>
>- /*
>- * Multichannel is not meaningful if max_channels is 1.
>- * Force multichannel to false to ensure consistent configuration.
>- */
>- if (ctx->multichannel && ctx->max_channels == 1)
>- ctx->multichannel = false;
>-
> return 0;
>
> cifs_parse_mount_err:
>@@ -1955,6 +1984,8 @@ int smb3_init_fs_context(struct fs_context *fc)
>
> /* default to no multichannel (single server connection) */
> ctx->multichannel = false;
>+ ctx->multichannel_specified = false;
>+ ctx->max_channels_specified = false;
> ctx->max_channels = 1;
>
> ctx->backupuid_specified = false; /* no backup intent for a user */
>diff --git a/fs/smb/client/fs_context.h b/fs/smb/client/fs_context.h
>index b0fec6b9a23b..7af7cbbe4208 100644
>--- a/fs/smb/client/fs_context.h
>+++ b/fs/smb/client/fs_context.h
>@@ -294,6 +294,8 @@ struct smb3_fs_context {
> bool domainauto:1;
> bool rdma:1;
> bool multichannel:1;
>+ bool multichannel_specified:1; /* true if user specified multichannel or nomultichannel */
>+ bool max_channels_specified:1; /* true if user specified max_channels */
> bool use_client_guid:1;
> /* reuse existing guid for multichannel */
> u8 client_guid[SMB2_CLIENT_GUID_SIZE];
>--
>2.43.0
Cheers,
Enzo
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