Re: [PATCH 1/3] ksmbd: reduce smb direct max read/write size

From: Namjae Jeon <linkinjeon@kernel.org>
To: Tom Talpey <tom@talpey.com>
Cc: linux-cifs@vger.kernel.org
Subject: Re: [PATCH 1/3] ksmbd: reduce smb direct max read/write size
Date: Mon, 31 Jan 2022 10:07:18 +0900	[thread overview]
Message-ID: <CAKYAXd_kaQBYyj68-ijxnxt1VsZMj09Qovss1vuzDGdF3CsP2A@mail.gmail.com> (raw)
In-Reply-To: <9bd2b636-ca5a-df2c-49ab-db14b7b453f1@talpey.com>

2022-01-31 4:04 GMT+09:00, Tom Talpey <tom@talpey.com>:
> On 1/30/2022 4:34 AM, Namjae Jeon wrote:
>> To support RDMA in chelsio NICs, Reduce smb direct read/write size
>> to about 512KB. With this change, we have checked that a single buffer
>> descriptor was sent from Windows client, and intel X722 was operated at
>> that size.
>
> I am guessing that the larger payload required a fast-register of a page
> count which was larger than the adapter supports? Can you provide more
> detail?
Windows client can send multiple Buffer Descriptor V1 structure
elements to server.
ksmbd server doesn't support it yet. So it can handle only single element.
We have known that Windows sends multiple elements according to the
size of smb direct max read/write size. For Melanox adapters, 1MB
size, and Chelsea O, 512KB seems to be the threshold. I thought that
windows would send a single buffer descriptor element when setting the
adapter's max_fast_reg_page_list_len value to read/write size, but it
was not.
chelsio's max_fast_reg_page_list_len: 128
mellanox's max_fast_reg_page_list_len: 511
I don't know exactly what factor Windows client uses to send multiple
elements. Even in MS-SMB2, It is not described. So I am trying to set
the minimum read/write size until multiple elements handling is
supported.
>
> Also, what exactly does "single buffer descriptor from Windows client"
> mean, and why is it relevant?
Windows can send an array of one or more Buffer Descriptor V1
structures, i.e. multiple elements. Currently, ksmbd can handle only
one Buffer Descriptor V1 structure element.

If there's anything I've missed, please let me know.
>
> Confused,
> Tom.
Thanks!
>
>> Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
>> ---
>>   fs/ksmbd/transport_rdma.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/ksmbd/transport_rdma.c b/fs/ksmbd/transport_rdma.c
>> index 3c1ec1ac0b27..ba5a22bc2e6d 100644
>> --- a/fs/ksmbd/transport_rdma.c
>> +++ b/fs/ksmbd/transport_rdma.c
>> @@ -80,7 +80,7 @@ static int smb_direct_max_fragmented_recv_size = 1024 *
>> 1024;
>>   /*  The maximum single-message size which can be received */
>>   static int smb_direct_max_receive_size = 8192;
>>
>> -static int smb_direct_max_read_write_size = 1048512;
>> +static int smb_direct_max_read_write_size = 524224;
>>
>>   static int smb_direct_max_outstanding_rw_ops = 8;
>>
>