[Patch v3 2/6] cifs: Allocate validate negotiation request through kmalloc

[Long Li]

> Subject: Re: [Patch v3 2/6] cifs: Allocate validate negotiation request through
> kmalloc
> 
> On 4/18/2018 1:11 PM, Long Li wrote:
> >> 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 at microsoft.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?
> >
> > ib_dma_map_page will return an invalid DMA address for a buffer on stack.
> Even worse, this incorrect address can't be detected by
> ib_dma_mapping_error. Sending data from this address to hardware will not
> fail, but the remote peer will get junk data.
> >
> > I think it makes sense as stack is dynamic and can shrink as I/O proceeds, so
> the buffer is gone. Other kernel code use only data on the heap for DMA, e.g.
> BLK/SCSI layer never use buffer on the stack to send data.
> 
> I totally agree that registering the stack is a bad idea. I mainly suggest that
> you capture these fundamental ib_dma* reasons in the commit. There's no
> other practical reason why the original approach would not work.

Sure I will fix that.