[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 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.

> 
> Tom.