linux-kernel - Re: [Patch v3 2/6] cifs: Allocate validate negotiation request through kmalloc

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Date:   Wed, 18 Apr 2018 10:07:52 -0400
From:   Tom Talpey <tom@...pey.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "longli@...rosoft.com" <longli@...rosoft.com>,
        Steve French <sfrench@...ba.org>,
        "linux-cifs@...r.kernel.org" <linux-cifs@...r.kernel.org>,
        "samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Cc:     "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [Patch v3 2/6] cifs: Allocate validate negotiation request
 through kmalloc

On 4/18/2018 9:08 AM, David Laight wrote:
> From: Tom Talpey
>> Sent: 18 April 2018 12:32
> ...
>> On 4/17/2018 8:33 PM, Long Li wrote:
>>> From: Long Li <longli@...rosoft.com>
>>>
>>> The data buffer allocated on the stack can't be DMA'ed, and hence can't send
>>> through RDMA via SMB Direct.
>>
>> This comment is confusing. Any registered memory can be DMA'd, need to
>> state the reason for the choice here more clearly.
> 
> The stack could be allocated with vmalloc().
> In which case the pages might not be physically contiguous and there is no
> (sensible) call to get the physical address required by the dma controller
> (or other bus master).

Memory registration does not requires pages to be physically contiguous.
RDMA Regions can and do support very large physical page scatter/gather,
and the adapter DMA's them readily. Is this the only reason?

Tom.