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

[Tom Talpey]

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@xxxxxxxxxxxxx>
> > > > >
> > > > > 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.

Tom.