> 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@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. Sure I will fix that.