> 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 <lon...@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.
