linux-kernel - Re: [PATCH 1/2] cifs: client: force multichannel=off when max

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Message-ID: <CAH2r5mu9xUQz5e1Mf-dBCNh2_y2jnxPYMhmuHr1bVqKC6atd8w@mail.gmail.com>
Date: Mon, 22 Sep 2025 11:14:14 -0500
From: Steve French <smfrench@...il.com>
To: Enzo Matsumiya <ematsumiya@...e.de>
Cc: Henrique Carvalho <henrique.carvalho@...e.com>, rajasimandalos@...il.com, 
	linux-cifs@...r.kernel.org, sfrench@...ba.org, pc@...guebit.org, 
	ronniesahlberg@...il.com, sprasad@...rosoft.com, tom@...pey.com, 
	bharathsm@...rosoft.com, linux-kernel@...r.kernel.org, 
	Rajasi Mandal <rajasimandal@...rosoft.com>
Subject: Re: [PATCH 1/2] cifs: client: force multichannel=off when max_channels=1

. >Do we even need ->multichannel flag at all?

Yes - especially in the future.   The goal is for the user to have
three options:
1) (preferred) "multichannel" (max channels will be dynamically
selected and can change) the client gets to choose how many channels
to connect to based on what it sees in the output of the most recent
query interfaces call (it can change on the fly as server dynamically
adds and removes channels or networks become temporarily unreachable)
2) "max_channels="   This is for the case where user wants to choose
the number of channels rather than have the client automatically
(hopefully soon in the future) choose it for you
3) if server has mchan bugs, allow client to mount with no
multichannel (or equivalent max_channels=1)

But ... "remount" also has to work for the three above (and currently
doesn't) and this is what I am hoping the recent patches can fix (?)
but haven't tested them enough yet

> I'd actually like to propose going even further and having the whole
module behaving as if multichannel was always on, even with
max_channels=1

Obviously the goal (would love patches for this) is to turn
multichannel on by default, have the client select the appropriate
number of channels by default etc. but we have to let the user
override it (as described above)

On Mon, Sep 22, 2025 at 9:59 AM Enzo Matsumiya <ematsumiya@...e.de> wrote:
>
> On 09/22, Henrique Carvalho wrote:
> >Hi Rajasi,
> >
> >On 9/22/25 5:24 AM, rajasimandalos@...il.com wrote:
> >> From: Rajasi Mandal <rajasimandal@...rosoft.com>
> >>
> >> Previously, specifying both multichannel and max_channels=1 as mount
> >> options would leave multichannel enabled, even though it is not
> >> meaningful when only one channel is allowed. This led to confusion and
> >> inconsistent behavior, as the client would advertise multichannel
> >> capability but never establish secondary channels.
> >>
> >> Fix this by forcing multichannel to false whenever max_channels=1,
> >> ensuring the mount configuration is consistent and matches user intent.
> >> This prevents the client from advertising or attempting multichannel
> >> support when it is not possible.
> >>
> >> Signed-off-by: Rajasi Mandal <rajasimandal@...rosoft.com>
> >> ---
> >>  fs/smb/client/fs_context.c | 7 +++++++
> >>  1 file changed, 7 insertions(+)
> >>
> >> diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
> >> index 072383899e81..43552b44f613 100644
> >> --- a/fs/smb/client/fs_context.c
> >> +++ b/fs/smb/client/fs_context.c
> >> @@ -1820,6 +1820,13 @@ 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:
> >
> >Do we even need ->multichannel flag at all? Maybe we could replace
> >->multichannel for a function that checks for max_channels > 1?
>
> I agree with Henrique.
>
> I'd actually like to propose going even further and having the whole
> module behaving as if multichannel was always on, even with
> max_channels=1 -- the only difference being the need to run the
> query_interfaces worker.
>
> This is probably work for another patch/series though.
>
>
> Cheers,
>
> Enzo
>


-- 
Thanks,

Steve